FWIW, for several years I've tried backuping my gmail account with imap (including some stuff made specifically for gmail): It never succeeded. The best syncer were running for one month, and after one month it hit some mails that it simply couldn't retrieve? Like I guess it was in too cold storage and timeout-ed? I don't know.

So I can understand why using Google's proprietary API might work better (or not, I don't know)

Anyway, as a sibling says, nowadays Google Takeout includes mbox and work properly (and is pretty fast, like half a day), but doesn't allow continuous update.

And I migrated to another mail provider (infomaniak), and I've thanked myself for using my own mail domain name years earlier.