Disclaimer;:My experience on this is from 10+ years ago on HTC’s Legend, Moto’s Droid 3, and Samsung’d S5.
Back in the day, I’d switch between custom ROMs and stock by simply booting into TWRP (or it’s predecessor which name’s I forgot, probably CWM or Philz) and restoring a backup.
Switching between backups was easy enough, unrooting/unhacking the locked bootloader not so much.
Obviously, anything you did inbetween said backups was gone for non cloud based apps.

I imagine if you have a phone with a proper bootloader unlock it’s likely easier than one which needed to bootstrap itself using kexec or other shenanigans.