jasnxl However, I'm hoping there's a straight forward way to backup all of the phone's data under stock Android, and then restore it under GOS.
That would be great, but there isn't.
jasnxl If the GOS project made the seedvault app available to be installed onto stock Android phones, (if that were possible), that should be the easiest method. It's just not clear how to install it on stock Android phones;
For it to work, it would need to be a system app. In the Android model only the OS author (Google, in the case of the stock OS) can install a system app.
jasnxl
OAndBackupX (Neo-Backup)
Migrate
Linux Android Backup - Companion App
I believe each of those either requires root access or can't back up everything.
Unfortunately, this is not an easy problem to solve, and it has not been solved.