While I agree it's nice to have access to nixpkgs' packages in other OSs (I've never did this so take the following with a grain of salt), it is my opinion that you're missing out on the biggest features if you don't fully opt for the nix approach.

I wouldn't reduce the nix tools to a package manager. It's a set to interact with the nix language, which primarily is a language to build a system from. You have the biggest advantage when you know that your system only consists of components built from your set of instructions (of course this pulls in a lot of stuff from nixpkgs) because that brings your system closer to reproducibility. It also makes it more consistent.