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
Yeah that seems like an area for improvement in Godot's GUI system indeed. For my stuff, I never had a need for theming, but good to keep in mind if one ever wanted to do an app shared with others.
Another homegrown personal tool I had once written was in Go with DearImgui, so basically with AllenDang/giu... also really quite rapid dev and lean-enough binary / perf.
But Godot is wysiwyg for GUI stuff, so that's a plus..
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