So the answer to your question is yes. It is possible and kind of required to go the full nix route with NixOS. It might not always be super straightforward with large DEs, and for sure works much better with window managers that already utilize text configuration.
It's really nice. When you setup a new system, you don't have to spend hours changing settings, configs, and installing packages. With NixOS, just copy the config file from your old system and then run nixos-rebuild.
I really wish the default template switches to flakes soon. Using flakes is a must for the best experience, and converting the default config is at best a hassle and for newbies a huge mountain to cross.