There’s already an existing feature suggestions addressing this request.
As you already pointed out… this can already be handled using a tv-show type library – in many cases it’ll work right away with the Plex TV Series agent – where it doesn’t there’s various 3rd party agents focusing on special sources for anime related metadata.
If that’s not sufficient for you I recommend voting/commenting in that thread in order to avoid distracting/dispersing votes.
Therefore closing this thread as a duplicate.
