Unfinished and hacky, please don't judge

gistfile1.txt

{ pkgs, lib, config, ... }: let
  i3Config = pkgs.writeText "i3-minimal.conf" ''
    # i3 config file (v4)
    set $mod Mod4
    floating_modifier $mod
    font pango:monospace 8
    bindsym $mod+f fullscreen toggle
    default_border none
  '';
  cfg = config.services.vnc-thinclient;
in {
  options = {
    services.vnc-thinclient = {
      user = lib.mkOption {
        type = lib.types.string;
        description = "User to run the VNC client as";
        default = "vnc";
      };
      server = lib.mkOption {
        type = lib.types.string;
        description = "VNC server to connect to";
      };
      extraOptions = lib.mkOption {
        type = lib.types.list lib.types.string;
        description = "Extra command-line options passed to vncviewer";
      };
    };
  };
  config = {
    users.users.${cfg.user} = {};
    services.xserver = {
      enable = true;
      displayManager.auto = {
        enable = true;
        user = cfg.user;
      };
      windowManager.i3.enable = true;
    };
    systemd.services.vncviewer = {
      wantedBy = ["multi-user.target"];
      after = ["display-manager.service"];
      path = [ pkgs.tigervnc pkgs.coreutils pkgs.xorg.xset ];
      serviceConfig = {
        User = cfg.user;
        Restart = "always";
      };
      script = ''
        set -x
        export \
          DISPLAY=:0 \
          XAUTHORITY=${config.users.users.${cfg.user}.home}/.Xauthority
        until xset -q >/dev/null; do sleep 1; done
        vncviewer \
          -FullScreen \
          -AlertOnFatalError=off \
          ${lib.escapeShellArgs cfg.extraOptions} \
          ${lib.escapeShellArg cfg.server}
      '';
    };
  };
}