I know you can build a Debian system with debootstrap. Using debootstrap it should be possible to create a custom image. The main partition could be read only with separate mounts for anything that need to be read write.
Using containers it should be possible to create a filesystem image. I think the tricky part it testing the image and then updating the existing partition. Maybe some custom ostree tool could do the trick. If not there is always rsync and btrfs snapshots.
uBlue is good, but only if you follow the official templates. I was following some other thing which did things very differently and my custom iso ended up broken i.e Anaconda was crashing and installation was impossible.
Edit: the thing is called Bluebuild. I’d recommend to steer clear of Bluebuild and just using the official template on Github. I’m still yet to do that myself but it seems like it might actually work, unlike Bluebuild.