Thanks, I can confirm running `RotT.x86_64 -screen-fullscreen 0` from terminal launches the game in windowed mode, and then the resolution can be changed in game settings. I could perhaps make a script to do that, since I didn't see a way to launch with options from itch.io itself. Maybe I can with Lutris though.
Solus is a rolling distribution, I'm up to date, using the Budgie flavor.