Yeah, Home Manager is more redundant if you're using Nix-Darwin for a configuration.nix. Non-NixOS Linux doesn't have an equivalent module system to Nix-Darwin, though, so if you want a module system for declarative config there it's Home Manager.
There is some work in NixOS-WSL on a special Systemd shim which allows that NixOS instance to work with the WSL-provided Systemd that Microsoft recently added with their 'native' systemd support. I wonder if that can be extended to port NixOS to systems where it doesn't control the whole stack.
Ideally we'll eventually see existing module systems unified with that of NixOS via some kind of process management abstraction framework, and you'll be able to do NixOS-like things in more places with a shared codebase.
Edited to add: there's nothing as simple as NixOS or Nix-Darwin, but you can use Disnix to deploy configured services via Nix on non-NixOS. Not something I'd go for on desktop, but it's there.
