This plan is absolutely fantastic IMO.
I tried this week to get started with a Pi 4 and was successful but it annoyed me that I couldn’t use my standard setup and had some problems with Flakes.
I’m going to test if the UEFI approach works better, at least I’m familiar with it 
.
The firmware part could be quite easy no?
Flash an image with a small partition and that’s that? Looks like that’s exactly what you have to do with Tow-Boot💪.
The cattle part sounds interesting as well but I haven’t tried yet to build an image myself.