Very nice submission from you!
One thing I noticed is, that your UI has the mouse propagation enabled, what you probably dont want. The first interaction I had, was me opening credits, clicking on the credit frame at the position of the exit menu button and suddenly my game froze (which was obviously the get_tree().quit()). So I obviously tested it with the ingame UI and it also lets mouse clicks through.
Visually very pleasing, I liked the art and style of it, great job!