After seeing that services.xserver.displayManager moved to services.displayManager, how to set monitor settings?
# Enable the X11 windowing system.
services.xserver.enable = true;
services.xserver.xkb.layout = "us";
services.xserver.xkb.variant = "";
services.desktopManager.plasma6.enable = true;
services.displayManager = {
sddm.enable = true;
sddm.theme = "breeze";
defaultSession = "plasma";
setupCommands = ''
${pkgs.xorg.xrandr}/bin/xrandr --output DP-1 --primary --mode 3840x2160 --rate 60
'';
};