I still can't believe it's not more widely used (and I only started using linux full-time earlier this year). It confines the entire (non-user) filetree into the read-only /nix directory, and manages every single component of the OS through Nix, the package manager - and I guess nix-daemon in the case of NixOS specifically - 100% declaratively. You define the entire thing through a single configuration.nix file.
You can even do crazy stuff like erasing the root upon each reboot, leaving only /nix and /home if you wanted, and I think I remember in the article I was reading about it that it can mount everything on a tmpfs or something like that, so you have a perfectly "clean" root tree every time you restart the computer (as /nix is stateless, it's guaranteed not to change except when rebuilding the system configuration).
The point of nixos is really the declarative aspect, with "splitting" the OS being more of a secondary benefit, but for your case, you might like the whole declarative builds thing in general to accomplish that.