Google Takeout will export all your mail in mbox format which is easily handled by mail programs like Apple Mail and Thunderbird and should let you sync everything to a new service (if that service won't let you transfer it themselves). I'm using iCloud+ from Apple. It's cheap at $1/mo for 50GB of space and lets me have my custom domains. $3/mo will get you family sharing and 200GB of space (if you want multiple acco…
I just did the Gmail —> iCloud transition and was frustrated that the MBOX format failed to maintain labels / folders. Everything went into a single place. On researching it seemed like the best process was to set up two IMAP accounts in Apple Mail and simply drag and drop the directories. I’ve done this, but it is still a pain and took 100x longer than I expected.
That was way before the Catalina Apple Mail data loss bugs. https://mjtsai.com/blog/2019/10/11/mail-data-loss-in-macos-1...
Any sysadmin will tell you Apple Mail has had occasional weird dogshit bugs for a decade.
Use Thunderbird, it's more likely to be robust and not corrupt your emails, then feel free to switch back to Apple Mail once you've moved everything.