Dynamic Category lists with nested referenced items

Righto, let’s say I’ve got a Collection of “Tutorials” and another collection of “Tutorial Categories” and the Tutorials reference the Tutorial Category it’s associated with, I’m trying to work out how to create a dynamic dropdown list that has "Category > Items referencing said Category pretty much exactly like the Lesson Library side-nav in WU.

I thought it would be as simple as a nested Collection List but the nested collection list needs to reference each Tutorial individually which is going to be a pain in the arse going forward.

So I basically want the Dropdown Toggle to display the Category name, then the Dropdown List to display the Items that reference the Category.

Unfortunately I cant share the project due to an NDA, but any help would be appreciated massively.