Hi!
I also spotted the new the @Menu annotation quite recently. While I added support for it to Viritin’s MainLayout this week, I made similar notes as you, and also that the “order” is double (int would have been easier for Java developers🤷♂️).
I don’t think you can alone use the Menu to automaticaly build hierarchical menu. For the Viritin’s similar (and much older) @MenuItem, I added parent property. That can refer to an empty marker class (then just simple grouping) or an actual route (then cliking to the parent navigates there). There I sort the next level always separatly → the simple int based order hint works very well there.
Another post I made earlier todo discusses the topic and links to a live demo: