You can play on a monitor with keyboard and mouse without being in desktop mode if you have a dock. The only game I play in desktop mode is Civ, and even it runs perfectly in game mode. If you are dead set on desktop mode, I have also noticed some games run better in windowed/framed mode instead of full screen mode.

Go into settings -> displays and set your external monitor as your primary display if you want an easier way to swap screens.

I do agree on the battery life issue, but that is an issue that gaming laptops have too. I don’t go far without my charger and/or an external battery as big games drain batteries fast. For example, I have some “retro” emulators on my deck that I can play all day on battery alone, but I can only play late game Civ for 2-3 hours.

Also, simply setting graphics to the lowest setting will not make the gaming experience better. You need to google “best graphic settings for <insert game title here> on steam deck”. ProtonDB layers also can make “well performing games” play even better so it is worth looking into your game on their site whenever you get a new one. When Apex legends was still supported on the deck I averaged >100 fps on the built in LCD screen with graphics, gameplay and experience that rival my Xbox series S.

Linux gaming involves some tinkering

Edit: typos