You could go with one documentary library for movies and tv-shows each and assign those “topics” as their own collection. This way, the “Ancient Egypt” tv-shows will show as related content in the same-named movie collection.
That being said… there’s already an existing suggestion discussing for Plex to support libraries that have mixed content (tv shows and movies). Please comment/vote in that thread in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.
[edit: and too slow…]
