Shouldn't this be "imap to sqlite" or something? Why tie it to one specific email provider?
IMAP is much harder, and much slower, and is bound by Google's bandwidth limits.
Maybe there have been times when it was broken or under high demand though?
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.
I installed a third-party client (Thunderbird, but I imagine any would work) on a local box, signed in with both emails, and just copied the mail over from one to the other. Low-tech, but it worked quite well. I may have forced some local cache/download for the original email, but I can't recall. I'll check later if it preserves headers and the like. I assume it would, but it wasn't that important to me.
I actually thought about writing at some point about the process of getting off gmail and all the funny things I ran across.