The official control scheme from the publisher has some issues with button prompts not showing in the Steam Deck OLED's native resolution. Mouse cursor also doesn't show. Changing to Gamepad with Joystick Trackpad and enabling "non Steam Controller" in the in-game options, fixed most of it.

I then edited the controller layout even further. Turned Left Trigger into a layer swap, to add more abilities to the face buttons while LT is held down (kind of like the Final Fantasy 14 controller layout).

The game is perfectly playable if you tinker with the control layout.

After a few hours playing, the game starts having stutters here and there. Fixed by rebooting the game.