Thanks for being so thorough. I can see the usefulness of some of the distinctions here.

I'm reluctant to use the canonical "Harry Potter - J.K. Rowling" fandom as I think it could be all things to all people and lead to a mismatch of expectations between recipient and creator. It's in the "Books and Literature" category, so it could be reasonably taken to mean the books, but it's also widely used to mean "everything HP related in all media".

Once they exist in one category (even non-canonically), tags can't ever be used in a different category, so a canonical freeform can't also be a fandom tag for our purposes.