I don't think this exists, @seabass. I am #fediverse / #ActivityPub / #Mastodon n00b, but I think your options for this are:
* Create a separate account for other interests & link them via your profiles (I see this often.)
* Contain the discussion to a thread (always reply, never new post) & use the appropriate hash tag.
* Use the hashtag as content warning so follows can ignore easily if not interested.
I post about #MST3K enough I've considered making @bkuhn@mst3k.social myself…