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'm using NixOS, it's really freaking awesome and super repeatable but it's also like smashing myself in the face with a brick every time I want to do something slightly unusual.
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.
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.
Also, not Debian, but https://github.com/cleanroom-team/cleanroom is very a DIY unusable system. I didn't build (most) off it, but I use it on my daily driver and a couple of other machines.
This distro is just an AIO kernel image that will bootstrap everything in RAM. You can mount additional devices for data persistence (for example you can mount your storage in /var).
Interesting distro idea, I might give it a try. One note, your documentation shows root actions using the $ prompt instead of # without sudo. That might be a bit confusing to those that use the prompt to determine privilege level.