Again, you shouldn't need to do this after the Steam Deck's August 5 update, but if you're having any issues, these are the steps to follow.
You can do this back in the Properties section of the game's page in Steam. On the General tab at the top you need to scroll down to the bottom of that page til you hit the Launch options. Enter '--skip-launcher' into the box without the quotation marks and you'll never see the Larian Launcher again and it will default to DX11.
Phew.