flake-utils-plus VS nix-processmgmt

You don't need channels to use nix-shell or nix-env. You can generate the Nix path and registry from your flake inputs. flake-utils-plus makes this easy with nix.generateRegistryFromInputs and nix.generateNixPathFromInputs.

Use flake-utils-plus to easily define multiple systems in flake.nix.and then use the --target-host flag of nixos-rebuild to deploy it remotely.

If you're trying to handle multiple systems with one config a flake is a significantly better option. I would personally recommend looking at flake-utils-plus and specifically this example config.

I looked at some past threads / examples and saw people recommend flake-utils-plus, but after reading the examples I don't understand how I would get home-manager on Darwin or Ubuntu.

Depending on your specific needs, the channelsConfig (modify arguments passed to Nixpkgs) and overlay tooling in flake-utils-plus may be adequate: https://github.com/gytis-ivaskevicius/flake-utils-plus/tree/...

For flakes you don't have to set up everything manually there are helper libraries, I use flake-utils-plus.

Converting to flakes is not that hard if you use a good library. The documentation is other people's configs, really.

Neat! I've been using flake-utils-plus's implementation until now. https://github.com/gytis-ivaskevicius/flake-utils-plus/blob/master/lib/repl.nix

I use flake-utils-plus based configuration to apply patches on top of nixpkgs where overlays don't suffice.

I somewhere saw this project being mentioned: https://github.com/svanderburg/nix-processmgmt

That's nice and it showcases how Nix can create a declarative process management atop a script-based imperative manager. How is your experience with it? Also note that there's https://github.com/svanderburg/nix-processmgmt, a manager agnostic processes management framework supporting s6 among others, but your way seems a bit more straightforward.

There's a project still being developed: https://github.com/svanderburg/nix-processmgmt my understanding is that if home-manager would use it (I don't know if there is even plan to do that, it's not the same author), it would be able to work without needing systemd.

(builtins.fetchTarball "https://github.com/svanderburg/nix-processmgmt/archive/6def8584c6b028c922c550859a07b989d21d6f73.tar.gz")

It's not really clear to me what you are asking. Are you maybe looking for nix-processmgmt? (NixCon 2020 talk: https://cfp.nixcon.org/nixcon2020/talk/TW79FU/)