My two cents:
- adb backup is unreliable at the best of times. I’ve seen enough backups that seemed successful at the time but cannot be restored. At the worst of times it’ll not work. Reinstalling apps from backup is reported not to work on Android 9 and higher. Adb backup as a whole is deprecated, meaning this won’t be fixed.
- it is difficult to verify or selectively extract data from an adb backup file, so if any part of your backup is not compatible with the new system you plan to install you will be unable to restore anything. It’s not possible to predict with certainty that the backup will be compatible.
- copying general settings over from one operating system version to another may lead to unintended side effects and instability that will take more time to diagnose than it would have cost to configure from scratch.