The comments I've seen have included Lemmy (caveat: apparently some concerns about the devs, default instance and policies) or kbin both of which are ActivityPub capable.
I doubt the API would be complicated, the issue will be in all of the little QoL/scaling features: moderator settings, automoderation bots, hosting photos and videos in a scalable way. I imagine behind the scenes bot/spam detection is an entire engineering team.