moviesandtv exist, but don't really allow people to discuss movies and shows, it's mostly about news.
I suggested at the time to just have a pinned post like "what have you been watching", but got banned by the mod for "backseat moderation" (see below: https://lemmy.blahaj.zone/comment/8381687)
The other issue is that as Lemmy posts don't have unique URLs that can be used by every instance, you can't just have a megathread with links to discussion threads, as that would work only for one instance