What DM do you currently use? Just startx?
It differs depending on the DM because the Xorg spec is mildly insane. If you ever want a headache go read the xdm man page (which is the foundation of all display managers), try to compare it against xinit (which is what startx calls internally), and then wonder how we arrived at those being alternatives.
The mpd module you use starts mpd with a dynamically generated configuration file in /run: nixpkgs/nixos/modules/services/audio/mpd.nix at d65bceaee0fb1e64363f7871bc43dc1c6ecad99f · NixOS/nixpkgs · GitHub
This means your mpd.conf is ignored. Just put the contents of it in extraConfig, it will be added to the bottom of this template: nixpkgs/nixos/modules/services/audio/mpd.nix at d65bceaee0fb1e64363f7871bc43dc1c6ecad99f · NixOS/nixpkgs · GitHub
You probably don’t want to add these to your packages, by the way, for 99% of libraries you need to use a nix shell to actually get them to link. Development stuff belongs in a shell.nix or flake.nix specific to the project. This also applies to the various X libraries in there, and probably some other things I overlooked.
Also, in general, you probably want to use .enable options for the absolute majority of packages, there are a bunch in your list for which I think such exist (e.g. mpd and mpdscribble). There’s no need to install them twice, and sometimes doing so can cause things to break due to PATH ordering and overrides.