Thanks :)

From your config:

    # proton GE needs to be installed imperatively using protonup

    environment.sessionVariables = {
      STEAM_EXTRA_COMPAT_TOOLS_PATHS = "/home/${config.globalConfig.user}/.steam/root/compatibilitytools.d";
    };

This is not the case anymore, there's now a proton-ge package. Mine is:

  programs.steam = {
    inherit (cfg) enable;

    extraCompatPackages = with pkgs; [proton-ge-bin];

    remotePlay.openFirewall = true;
    dedicatedServer.openFirewall = true;
  };

I'll take a deeper look into your config later, it seems quite nice, i could learn something from it.