That’s true for the configuration.nix. I still cannot fully wrap my head around using Nix Flakes for managing my nixos configuration, home manager and overlaying or creating packages. My setup so far works, but I still don’t feel like I fully understand it.
Unstable is and from what I’ve heard, nixpkgs even is one of the fastest repos with updating packages (also the largest). But I still wouldn’t recommend it for OP.