@mima some of that is pretty baked in, engineering choices made in the past that can't really be fixed with an update.
ActivityPub is fundamentally bound to instances, putting all of the power in the hands of instances instead of users.
So things like instance migration and moving posts and all require the cooperation of the instance that a person is leaving, which has little incentive to help out.
This instance focus is a criticism I have of ActivityPub, and there's just no real way to change it without building a whole different system.