Nix / NixOS

Howdy, I'm hoping someone can offer me some insight here because I've been stuck on this problem for an embarrassing amount of time.

I'm using flakes on a NixOS system. The basic outline of my flake.nix I lifted from this repo: https://github.com/notthebee/nix-config. I have one system I don't want to include home-manager, so I'm trying to make the home-manager configuration function (homeManagerCfg) in flakeHelpers.nix only be called if the extraModules list includes home-manager (stable or unstable or other).

I've tried everything I can think of so any help would be really appreciated here, thanks.