I was never able to integrate nix-bitcoin modules with nixos. I just use bitcoind from nixpkgs. Regular packages can be used by importing nix-bitcoin into your system configuration. But even things like hwi are upstreamed to nixpkgs at this point. Its not too useful to me. It seems like it was more designed for non-nixos use cases?
Discussion
Ah that makes sense.
Still figuring out how everything works together. Packages vs flakes especially.
Run it as a Flake. You can see my flake here: https://git.sovransystems.com