I love Godot and think it has a great deal of potential for this kind of thing. Sure using a game loop isn't really very efficient for basic UI application, but in your case that doesnt really matter too much. That being said, I really don't like how it handles UI themeing.

Buttons are a good example. If you want to define a button theme, you have to individually define the theme for every possible state the button can be in. No inheritance, the best you can do is copy and paste them over (which means if you want to tweak one, you have to tweak them all). Compared to something like CSS, its a nightmare to theme even slightly interactive Godot GUI's