Author here - one of the most notable facts about the app is that it's made entirely in Godot Game Engine! I think it's great for apps like this because it makes it especialy easy to iterate on GUI designs such as this one.
Feel free to ask me anything about Chord Chart Memo, or my experience with Godot.
I enjoyed your write up, and was surprised to read that it was built in Godot, which I'd only ever considered for games, but of course it makes sense that games are simply a kind of app.
Love that you employ the circle of 5ths in the keyboard, whereas a more naive approach might've been to just ascend stepwise from C.
Any plans for an iOS version? Does Godot make it easy to port to other platforms?