This kind of import is something that I would absolutely love to see, but some backend stuff has to be figured out. Unfortunately, importing and creating thousands and thousands of posts can absolutely hammer a server, and it gets amplified if everybody's doing it at the same time.

I had some ideas for a tool a while back that could import your posts first, help you sift through what your "Greatest Hits" were in terms of big life events or lots of conversation, and help you import those into another platform. The downside is, though, that you still wouldn't be able to reconstruct the threads for people who haven't moved over to the fediverse yet.