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.
Obviously, lightweight is open to some interpretation but Silverblue can be made very lightweight by simply uninstalling the default flatpaks. You’ll be left with a very basic GNOME shell and greeter, without any of the common GNOME extras. From there you could easily install your own window manager, greeter, and whatever apps you need.