This post is fair.
Nix is very flexible, and it hasn't yet stabilised on a firm set of recommendations for a happy path.
Going on a whim:
* Use nixos-unstable. It's defacto stable, and gets much more attention than nixos-stable.
* Use flakes.
* Don't use multiple versions of nixpkgs. In the rare case a package is failing to build, then raise an issue, or wait, or rollback.
* On NixOS, don't use user profiles. They won't interop in the way OP hopes.
* Only use nixpkgs. If you absolutely must use another flake, only use popular ones from https://github.com/nix-community.
But until the community give opinionated suggestions, users will stray towards bad practices.
(Also, no need to mix pipewire and jack. Pipewire can emulate jack.)