I've found it difficult to find somewhere to import an Mbox. You've gotta grab a desktop app like Thunderbird that supports it and then upload the data again. Easiest for me has been to just have any email client that supports IMAP on a desktop PC and open it occasionally so that it syncs. This saves you the Takeout delay of packaging up your mail, and ensures you're only downloading the delta since last time. I do have some Takeout snapshots though in case I ever absolutely have to go back.
