I've only changed the system to 40 fps to get a little extra battery juice out of the Steam Deck.

After initial full screen success, the game now launches into a smaller window frame. It can be successfully fixed after changing the window to either borderless or windowed, confirming, and then swapping back to full screen.

It's an easy fix, but one I'll tinker with to try and avoid in the future (and update if anything works).