I've seen reports of problems with the setup.
What could help is to go through the setup agent without signing in or restoring data, then update the iPhone 15 to iOS 17.0.2, reset it to factory settings and then do the proper setup with signing in and restoring.
Since your iPhone is stuck, I would recommend a hard reboot: Force restart iPhone - Apple Support (MY)
That will probably end up in an unsuccessful setup which lets you then proceed as described above. After two hours, I would not expect the migration to recover itself even when the amount of data is high.
Also, make sure, your old iPhone is on the latest version of iOS.