Interesting game, I think I may have some insight into the UI scaling issues with the game as I'm familiar with similar things in Unity. Certain aspects of the overlay don't scale correctly based on screen size, so text is offscreen, or there are huge black letterbox style bars that obscure the game, which requires switching back and forth between fullscreen and not fullscreen to see everything. If I had to guess, you have a canvas element set to constant pixel size/physical size, when it should probably be set to scale with screen size. I am unsure if this is the exact issue, but I think it's likely part of it.
Also, love the art and the character's animation!