Thanks for this. I am loosely familiar with mbsync and OfflineIMAP, used to use OfflineIMAP with mutt but it had a lot of bugs and crashed often.

In this article https://anarc.at/blog/2021-11-21-mbsync-vs-offlineimap/ the author observes missing emails with both of them.

And I thought neither tool supported upload, am I mistaken?

Is simply using rsync to transfer the mail folders from one Maildir-based email server to another a viable approach?

If you have file-level access to the mail servers, any kind of file transfer of the maildirs should work. But from your description I'd encourage you to spend some time looking at imapsync, I think it's the closest fit to what you're wanting to do.