You can use Nix on your Linux (Ubuntu, maybe Gentoo) for the projects you want without going full NixOS.
We use NixOS on three machines around the house, but I use Nix + home-manager on my MacBook, and various Ubuntu servers at work. It's nice that I can largely use the same configuration across all machines. Just a home-manager switch and I am in my native environment.
last time i checked the install-instructions on Catalina, you had to create an unencrypted volume to store the nix-things, which i'd prefer not to do (i understand those files are probably non-personal, so no need to encrypt them, but i don't want to have to remember that half of my hard-drive is unencrypted)
is that still required or is there a simpler approach now?
Ideally the Nix store would be in some non-root location. But that requires a completely new binary cache among other things (since /nix store paths are hardcoded in binaries, scripts, etc).
Also making /nix a symlink doesn't really work in some cases, since realpath reports the actual path and that may break builds/applications.
It used to work so nicely out of the box :(, but I can also understand why Apple wants to enforce read-only system volumes, since it blocks nastier rootkits, etc.
Honestly, I think they should just relocate it to a different location that can be persisted and rebuild the packages. I don't think people care about the location as long as they don't have to recompile every single thing. This could also be useful to catch all bugs where /nix was assumed.