here’s what Hultgren writes in his commentary:
There are several reasons for concluding that 1:3-4 is a pre-Pauline creedal formula: (1) The term “Spirit of holiness” . . . is a Semitism that is otherwise not used by Paul or any other NT writer. . . . (2) The work of the Spirit in this passage is not characteristic of Paul’s own thinking. Here the Spirit is instrumental in the resurrection of Jesus from the dead, whereas in Paul’s thinking it is God (the Father) who raises him from the dead, and the Spirit proceeds from Christ; it does not act upon him. (3) The Spirit/flesh antithesis, as expressed here, is uncharacteristic of Paul. Where an antithesis exists between the Spirit and the flesh in the letter of Paul, it is anthropological . . . not Christological. . . . (4) The Christological view expressed in the formula is not that of Paul. . . . Paul holds a three-stage Christology (preexistence, incarnate earthly existence, postresurrection reign). The formula of 1:3-4, however, sets forth a two-stage Christology (earthly existence, postresurrection reign) by which Jesus became Son of God only by his exaltation; the descendant of David was designated Son of God at his resurrection.
On this issue, Hultgren (2011:46) mentions three views:
(1) Some are forthright in saying that the unit expresses a primitive “adoptionism,” meaning that in his earthly life Jesus did not have a messianic status but became the Messiah/Son of God at his resurrection, thereby being “adopted” as God’s Son.43 (2) Others do not use the word “adoptionism,” but agree that the parallelism asserts that at his resurrection Jesus took on a status and role that he did not have previously.44 (3) Still others maintain that the passage does not say that the resurrection was the occasion by which Jesus became the Son of God; instead, it was simply the moment at which Jesus (whose preexistence and incarnation are presupposed) began his exalted life.45
and the footnotes here:
J. Weiss, Earliest Christianity, 1:118-19; 2:476; R. Bultmann, Theology, 1:50; O. Michel, Römer, 40; J. Knox, IB (Romans), 9:382; E. Käsemann, Romans, 12; F. Hahn, The Titles of Jesus in Christology, 248-49; H. Conzelmann, Outline of Jesus in Christology, 248-49; H. Conzelmann, Outline of New Testament Theology, 77; L. Goppelt, Theology of the New Testament, 2:22; C. Burger, Jesus als Davidssohn, 28-29; K. Wengst, Christologische Formeln, 115; G. Strecker, Theology of the New Testament, 68; R. Jewett, Romans, 104. C. K. Barrett, Romans, 22, says that the passage has an “adoptionist tinge”; J. Dunn, “Christology (NT),” ABD 1:983, notes that the passage has an “adoptionist” ring.

C. H. Dodd, Romans, 4-5; E. Best, Romans, 11; O. Michel, Römer, 38-39; Eduard Schweizer, “πνεῦμα,” TDNT 6:417; J. Dunn, Romans, 14; P. Stuhlmacher, Romans, 18; L. Keck, Romans, 45; M. de Jonge, Christology in Context, 49; C. Tuckett, Christology and the New Testament, 50. J. Scott, Adoption as Sons, 228, 234, 236, grants that “adoptionistic language” is used, but he says that it refers to Paul’s perception (“Paul perceived,” p. 243) on the Damascus Road that Jesus was the adopted Son of God in fulfillment of 2 Sam 7:12-14. That view, however, cannot be sustained, for Paul writes concerning something that happened to Jesus, not himself.

Origen, Commentarii in Epistulam ad Romanos, ed. Theresia Heither, 5 vols. (New York: Herder, 1990-95), 2:194; M. Luther, Romans, 147; A. Nygren, Romans, 48; L. Goppelt, Theology of the New Testament, 2:22-23; C. Cranfield, Romans, 58, 62; J. Fitzmyer, Romans, 236; idem, “The Christology of the Epistle to the Romans,” 85; M. Hengel, Studies in Early Christology, 157; M. Fatehi, The Spirit’s Relation to the Risen Lord in Paul, 248; D. Moo, Romans, 48 (“change in status or function”).
Sperma can be “offspring/descendant” as well as (actual) “seed,” in the same way that Hebrew זֶרַע can mean both, too. But “sperma of David” in Romans is almost certainly a reference to [2 Samuel 7:12] (which uses זֶרַע / sperma). So I guess it’s (ultimately) a Semitism in that sense.
Bart Ehrman believes so. In How Jesus Became God, 2014 he acknowledges it differs from Paul’s own Christology. Ehrman posits Paul’s reason for using the creed was to establish a connection to the earliest Christians with those in Rome to whom he is writing, to prove he is a true Christian. This only makes sense if it would be familiar to those in Rome. Ehrman argues Paul must affirm the fulfillment of Jewish Scripture as he is charged by his contemporaries as preaching an anti-Jewish Gospel.

From How Jesus Became God, 2014
“Unlike the rest of the first chapter of Romans, these two verses are highly structured and well balanced into two thought units, in which the three statements of the first unit correspond to the three statements of the second”
Below shows the proposed structure:
A1 Who was descended

A2 from the seed of David

A3 according to the flesh,

B1 who was appointed

B2 Son of God in power

B3 according to the Spirit of holiness by his resurrection from the dead.
━━━
Yes! Romans 1:3-4 does exhibit an early Exhalation Christology (ie: a “Low Christology”). It is also indeed a pre-Pauline creed that he was citing in his epistle. Paul’s epistles are our earliest Christian writings, so anything he cites himself would our best glimpse into the earliest Christian thought. So, this is one of the earliest Christologies we have recorded. I stress “one of” however, because again, as you mention, Philippians 2 includes a poem with a different Christiology. These two would then be our two oldest Christologies we have available. In regards to the more specific nature of Romans 1:3-4:
“That this is a pre-Pauline creed that Paul is quoting has seemed clear to scholars for a long time. For one thing, as we have just seen, it is highly structured, without a word wasted, quite unlike how normal prose is typically written and unlike the other statements Paul makes in the context. Moreover, even though the passage is very short, it contains a number of words and ideas that are not found anywhere else in Paul. Nowhere else in the seven undisputed Pauline letters does Paul use the phrase “seed of David”; in fact, nowhere else does he mention that Jesus was a descendant of David (which was requisite, of course, for the earthly messiah). Nowhere else does he use the phrase “Spirit of holiness” (for the Holy Spirit). And nowhere else does he ever talk about Jesus becoming the Son of God at the resurrection. For a short two verses, those are a lot of terms and ideas that differ from Paul. This can best be explained if he is quoting an earlier tradition. Moreover, this earlier tradition has a different view of Christ than the one that Paul explicates elsewhere in his surviving writings. Here, unlike in Paul’s writings, Jesus’s earthly messiahship as a descendant of King David is stressed. Even more striking—as I will emphasize in a moment—the idea that Jesus was made the Son of God precisely at his resurrection is also stressed.” (Bart Ehrman, How Jesus Became God, p.220-221)
So as we can see, the creed Paul cites here seems to actually disagree with more general Pauline theology. The Philippians 2 poem actually seems to reflect Pauline theology more accurately, and likewise, reflected a later Christology:
“The first Christologies were almost certainly based on the idea of “exaltation.” Christ, as a human being, came to be exalted to the right hand of God, where he was made to share in God’s status as a reward for his faithfulness. The earliest Christians – the earthly disciples themselves (or at least some of them: we have no way of knowing if they all “converted” to believe this about Jesus) –thought that this happened at Jesus’ resurrection, where God “made him” the Son of God (and thus the Lord, the messiah to come, the Son of Man, and so on). Later there were Christians who thought this exaltation occurred at his baptism, so that he was the Son of God for his entire ministry. The other type of Christology came a bit later. It was an “incarnation” Christology which indicated that Jesus was a pre-existent divine being – for example, an angel – who became a human being for the purpose of salvation. This was the view of Paul, at least insofar as the Christ-poem of Philippians 2 represents not just an earlier point of view but his own view as well. But this poem, and possibly Paul himself, represent a kind of transitional Christological view, because even though Christ was a pre-existent being who became human, at the resurrection God exalted him even higher than before. So this perspective combines elements of both an incarnational and an exaltation Christology.” https://ehrmanblog.org/early-christology-how-i-have-changed-my-mind-for-members/
Additionally, Ehrman believes Paul elsewhere speaks of Jesus explicitly as an angel:
Many people no doubt have the same experience I do on occasion, of reading something over and over and not having it register. I have read Paul’s letter to the Galatians hundreds of times in both English and Greek. But the clear import of what he says in Galatians 4:14 simply never registered with me, until, frankly, a few months ago. In this verse Paul calls Christ an angel. The reason it never registered with me is that the statement is a bit obscure, and I had always interpreted it in an alternative way. Thanks to the work of other scholars, I now see the error of my ways. In the context of the verse, Paul is reminding the Galatians of how they first received him when he was ill in their midst and they helped restore him to health. Paul writes: “Even though my bodily condition was a test for you, you did not mock or despise me, but you received me as an angel of God, as Jesus Christ.” I had always read the verse to say that the Galatians had received Paul in his infirm state the way they would have received an angelic visitor, or even Christ himself. In fact, however, the grammar of the Greek suggests something quite different. As Charles Gieschen has argued, and has now been affirmed in a book on Christ as an angel by New Testament specialist Susan Garrett, the verse is not saying that the Galatians received Paul as an angel or as Christ; it is saying that they received him as they would an angel, such as Christ… The reason for reading the verse this way has to do with the Greek grammar. When Paul uses the construction “but as . . . as,” he is not contrasting two things; he is stating that the two things are the same thing. We know this because Paul uses this grammatical construction in a couple of other places in his writings, and the meaning in those cases is unambiguous.
For example, in 1 Corinthians 3:1 Paul says: “Brothers, I was not able to speak to you as spiritual people, but as fleshly people, as infants in Christ.” The last bit “but as . . . as” indicates two identifying features of the recipients of Paul’s letter: they are fleshly people and they are infants in Christ. These are not two contrasting statements; they modify each other. The same can be said of Paul’s comments in 2 Corinthians 2:17, which also has this grammatical feature.” (Bart Ehrman, How Jesus Became God, p.251-252).
For more on the Philippians 2 poem being viewed the way Bart sees it:
“Especially in ancient Jewish and Christian apocalyptic thought, figures of great eschatological significance could be thought of as in some way having a heavenly “pre-existence.” So, given the strong conviction that Jesus is the eschatological redeemer, it would perhaps have been a logical move to think that he must also be ascribed a prior, heavenly status or existence, however that was understood. As for corroborating evidence from early New Testament writings, most scholars see Paul’s reference to Jesus as the one “through whom are all things and through whom we exist” (1 Cor. 8:6) as reflecting the conviction that in some way Jesus was the agent of creation as well as the agent of redemption. Most scholars also see a more figurative reference to the idea of Jesus’ pre-existence and “incarnation” in 2 Corinthians 8:9, where Paul tells his intended readers that Jesus “made himself poor, though he was rich, so that you might become rich through his poverty.” In Philippians 2:6, Jesus “being in the form of God” is clearly intended as in some way a contrast/comparison with his then “taking the form of a slave” in v. 7. If the latter represents his status and mode as a historical, earthly, human figure, then “being in the form of God” is surely best taken to represent a different and prior status or mode of being much higher than being human, which he chose not to exploit for his own advantage. This too suggests that we must imagine some notion of Jesus’ heavenly “pre-existence” behind the opening words of v. 6… Philippians 2:6-11 represents a two-part narrative recounting Jesus’ self-abnegation, followed by God’s consequent exaltation of him… Jesus is then portrayed as given God’s own unique name,” (Larry W. Hurtado, How on Earth Did Jesus Become a God?, p.102-103)