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.
I use Silverblue on my main machine. I want something lightweight for my other machine.
Well, uBlue can be as light as you want it to be.
Some other projects I’m thinking of:
Is it for a personal computer or a server?