Similar premise, but not as technically reproducible. You can create and manage your own custom images using the blue-os build systems, it's dead simple. Basically you're just writing a Dockerfile. Most of the filesystem is read-only like NixOS, but it acts like a traditional Linux system with FHS support.

It's that last part that made me switch to it from NixOS. I was tired of random things not working because of Nix's opinions on how things should work. I run Bluefin Linux with Fleek and get all the benefits I liked about Nix with none of the nonsense.

I'm not well versed in NixOS, but the team over at Jovian are all very helpful and are running a great project.