The lock screen and display manager are two separate pieces of software. The display manager runs before a user profile has been selected for graphical login, so it does not have access to your user desktop/display settings

My NAS isn’t big enough.

My NAS isn’t big enough yet.

Gotta keep a positive outlook

You can also use this lil guy, but by default you can’t install unfree software (not sure if that is different if you use flakes): nixpkgs.config.allowUnfree = true;

You forgot

nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
    "steam"
    "steam-original"
    "steam-unwrapped"
    "steam-run"
  ];