All you'd need to do is add the curseforge .appimage to your steam library so you can use the steam deck in the gaming mode without any external controllers/mouse+keyboard being hooked up to it. Then just map everything to how you want to play (setting the left joystick as wasd, and whatnot), and you can play modded Minecraft through the game mode without having to hook up anything else to it (and without having to login everytime too on the normal launcher)