Something like… Category Groups - Theme component? It wouldn’t be hard to come up with category groups to contain everything, though we’d lose the rollup functions of a parent category with subcats.
I’m not having trouble with that, but it’s probably not super intuitive. If the default setup had parent categories in the sidebar list, then setting them all to “Show subcategory list above topics in this category” / “Boxes”, a la Documentation, could be nice:
(Apologies if I’m duplicating suggestions – there’s a lot to scan in this topic.)