You’ve summed up my whole entire experience with NixOS. I too would love a simpler Nix, and couldn’t quite commit to the time to learn everything. As a compromise I’ve been content to continue with nixpkgs and nix-env on Pop!_OS.
Having said, I’ve noticed a LOT of new nix flakes being added to System76’s new Rust based COSMIC desktop environment’s packages. And I’m hearing talk of immutability. Maybe we’ll get our wish.