I haven’t used Ansible but it sounds pretty much like that, basically you write out all the packages you want in a config and it builds the system from that. Very nice in terms of stability and maintainability. I’m very much an amateur so I can’t say for sure but I think the unused package issue would still exist on nix.

I’m just getting started with nix, if I’m understanding it correctly I think that is kind of what nix package manager does? It keeps packages and their versions separate and doesn’t delete them, so that you can update some programs and their dependencies without breaking other programs that depend on other versions of those same dependencies. https://www.linux.com/news/nix-fixes-dependency-hell-all-linux-distributions/