... as well as an overall toggle to turn on/off notifications for the entire category.
Tags has an overall toggle which is actually three radio buttons, with Off not being one of the
That gives me an idea.
Instead of an on/off togle for the category, a compromise might be a one-of-four selector: All Groups, Groups I Own or Moderate, Groups I Own, or Off. That probably covers most use cases for people more interested in some groups than others, without needing a toggle for each subscription.
I think I'd still rather have a toggle per group in each category (or maybe that should be a toggle per category in each group), but I can see where that might get a bit cluttered.