Sorry for the long wait, but the only solution I could find after moving to Linux Mint was to just autohide my toolbar and play windowed. I definitely feel you should still add some resolution options at some point for the sake of accessibility. Looking forward to finally trying this! I'm a big Megaten fan so this should definitely scratch the itch (no pun intended).
No worries on the delay, I'm just glad you were able to find at least a partial fix.
Regrettably, most of Demon's core systems (including those related to rendering and resolution) were made by me back when I was much newer at engineering (over a decade ago now!): while it would not be impossible to refactor them to something capable of better resolution support, it would be a pretty significant undertaking that I admit is unlikely at this point given Demon is no longer in active development.
I apologize for this and I hope you are able to enjoy the game in spite of the resolution limitations. I am also a big Megaten fan, which is what inspired me to make this game in the first place. :D Cheers, and good luck in the tower!