According to soul sleep advocates, did the Apostle Paul ever abandon his Pharisaic beliefs about the afterlife after becoming a Christian?

Evangelical theologian Michael F. Bird wrote in a 2019 blog article The Intermediate State: What the Bible Tells Us that Paul wasn't very clear about the intermediate state. But from what we can infer from his writings, it appears that Paul never embrace Christian mortalism. Quotes from the article (emphasis mine):

References to an intermediate state were not a mainstay of Paul’s eschatological teachings that focused primarily on Christ’s parousia, the resurrection, and the final judgment. Information about an intermediate state must be inferred from Paul’s remarks elsewhere. Paul writes to the Philippians from Ephesus about his imprisonment and possible execution:

Phil 1:20-24

Paul contrasts “living in the body” with departing to “be with Christ, which is better by far.” Paul provides no data about the nature of this state, where it takes place, or what form he exists in there, and we can only assume that death entails a removal from his body and transportation to instant intimacy with the Savior.

The place where Paul discourses specifically about the postmortem fate of the individual, starting with himself, is

2 Corinthians 5:1 – 10:

It is often alleged that in these verses Paul has abandoned the apocalyptic eschatology of 1 Corinthians 15, with its future resurrection of the body, for a resurrection into a spiritual body into God’s presence immediately after death. But this is hardly likely since Paul’s reference to “we know” (5:1) introduces a rehearsed doctrine rather than a newly fashioned one. Second, Paul has intimated earlier in the letter his continued affirmation of the resurrection (1:9 – 10; 4:14) and affirms it again a few sentences later (5:15). Third, 1 Corinthians 15 and 2 Corinthians 5 share a lot of vocabulary, such as “unclothed” and “earthly.” Paul’s teaching on the future remains consistent, though in 2 Corinthians 5 he does begin to talk about the immediate postmortem fate of the individual, starting with himself.

Paul had intimated an interval between death and resurrection that was a bodiless one (1 Cor 15:35 – 38) and a temporary state (15:32 – 44). Now as he faces the expectation of death ahead of the parousia, he turns his mind to what lies in store for him. If Paul expected to receive a spiritual resurrection body after his death, it leads one to wonder why he would still anticipate the Lord’s return in the future since resurrection and parousia have been consistently bound together in his eschatology across the Thessalonian and Corinthian correspondences and also later in Philippians and Romans.

What Paul appears to envisage immediately upon death is not a spiritual resurrection, but a future spiritual mode of existence that is transcendent, yet not fully actualized until the parousia. There is a transition from the sarkic (fleshly) and somatic (bodily) form of existence into a heavenly dwelling in the company of the Lord, characterized by a heightened form of interpersonal communion with Christ.

Yet this state is clearly something that is prior to Christ’s parousia and the resurrection because it is ahead of the judgment of believers when their resurrection will take place. Paul hopes to please the Lord in both his bodily state and in his heavenly dwelling, knowing that he will stand before Christ at the final judgment. In any case, the promise of the Spirit and the object of faith is such that he looks forward to leaving his body, imagining a time away from the body in this eternal dwelling, and then presumably being raised to stand at the final judgment.

...

It’s important to note that an affirmation of a future resurrection does not demand that there is no conscious existence in a nonbodied, postmortem state ahead of the resurrection. When Paul dies, he intends to be with Christ, which is better than his current bodily existence (Phil 1:23); yet he also thinks of the immediate postmortem state as something temporary, like a car on loan from a mechanic, waiting for the original vehicle to be renewed (cf. 1 Cor 15:35 – 38). So it seems that upon death, the separation of body and soul is both blessing and a bummer, something enjoyable but also somewhat ephemeral. The unity of the material and immaterial parts of one’s being are the norm, but death ruptures that norm ahead of the resurrection. Yet, despite the awkward disunity of body and soul at death, believers still enjoy God’s presence and look forward to the day when they will be raised in a psychosomatic unity of body and soul in God’s everlasting kingdom.

Christ is the place of rest

It is difficult to plot the exact place and type of existence in the intermediate state. No text, save perhaps 2 Corinthians 5, discourses on it at length. But overall it seems that Joachim Jeremias was correct when he writes: “The New Testament consistently represents fellowship with Christ after death as the distinctively Chris tian view of the intermediate state.”

The intermediate state has to be articulated primarily in christological terms. Paul is clear that one departs to be with Christ (Phil 1:23), and according to John the Evangelist, where Christ is, there believers will also be (John 14:3). For nothing, not even death or demons, will separate believers from the love of God that is in Christ Jesus our Lord (Rom 8:38 – 39). The intermediate state brings fellowship with Christ, and in him we find also the continued fellowship of believers ahead of the final consummation (Heb 12:23). Death does not eradicate the believer’s union with Christ or communion with fellow believers. Whatever life is ahead in the eschatological future, interim and final, it can only be a “life in Christ.”