How to set lower than native resolution for fullscreen?

[Moshugan]

I can't seem to set a lower resolution than the monitor's native resolution if I wish to have the game in fullscreen. If I set a lower resolution, then the game is displayed windowed. This is especially troublesome for Steam Deck. The game runs fine with the default 1280x800 resolution of the Steam Decks screen when using FSR Ultra Performance. The framerate is something like 30-40.

I would love to try a lower resolution with FSR but I can't change the resolution of the game since Steam forces in to display in fullscreen in Gaming Mode. Is there any way to set a lower fullscreen resolution than native?