After the resurrection. Which adoption established Jesus in even higher status than he held before. Hence he became God’s heir (not merely his agent), and thus “the firstborn of many brethren.”

When did the adoption by God occur?

Where does Paul say Jesus only acquired that specific power after he died? You seem to be confused. The Philippians creed says Jesus gave up that power to effect the atonement sacrifice, and then God exalted him back to his previous, and indeed higher, status, in result. This is also clear in 1 Cor. 15, where Jesus is granted greater powers than previously for his sacrifice, which powers now permitted him to defeat the Devil, which power he had not had previously, explaining the evils of all human history. By contrast, Paul says Jesus’s resurrection is now “the firstfruits” of the end-times resurrection, which cannot mean he was resurrected thousands of years ago. It can only mean he was resurrected recently, and that in fact that being the case was proof the end was nigh.

The crucifixion must occur before creation.

Paul viewed Jesus as a demiurge that created the world on God’s behalf.

Jesus only obtained that power after he died and was adopted by God.

I doubt it. He speaks as though it was a later-age event, overthrowing Satan’s thrall only recently. The cosmic symmetry of it requires it to have just happened to overthrow Satan’s long reign. Similarly, Paul says Jesus was the firstfruits of the general resurrection, meaning he regarded Jesus’ sacrifice as beginning the imminent end of the world. He cannot have said that of an event that predated even Creation.

It’s permanently available for free: e.g. here.

Greets and thx