Also, I'm curious as to what you mean by "system package manager" when nix explicitly supports installation of software by unprivileged users (the packages only get linked to from the users profile, so it's safe). I also run nix under debian a lot very successfully.

If you want the game to load/unload addons at runtime, rather than just wrap the games executable with flags, then yes you have to learn to talk to nix daemon directly which is a bit more work. But do that and things will still work just fine.