The command line xcodes is already packaged and available at https://search.nixos.org apparently by community volunteers. Likely still pointing to the old repo that has moved under XcodesOrg.
Ideally, it would be nice if the authors of xcodes and XcodesApp embraced nixpkgs for each and maintained them alongside Homebrew.
Yes, nix-darwin can control Homebrew but I like to only use it when there's no other option.
Dealing with the Nix Community may take considerable time to get your packages published officially on nixpkgs. However merely creating your own flake.nix for xcodes & XcodesApp along with appropriate documentation may be the best way forward. Users of nix-darwin would be able to add your flake.nix straight off your repos directly without pulling from nixpkgs.
Thank you for your consideration.