Meta makes their own nice, QoL-rich instance that could integrate with Facebook/Instagram.

This part could actually be enough on its own, TBH. Imagine that there's one Fediverse instance where you can interact with the rest of the Fediverse and interact with FB and IG, but it doesn't propagate stuff between the two networks (i.e. it doesn't allow people on Beehaw to see what someone on FB posts, and vice versa). Now there's a reason for everyone to migrate to Meta's instance, and a built-in way for Meta to advertise the migration to everyone in the FV. Once it sucks up enough users, it just de-federates from everything else and goes on its own way.