I have been looking into this. There is mbsync (isync)[1] which will let you download a imap account. There is also a premade docker image[2] which you could trigger via cron and sync your mails. For gmail it is recommended to get a login token but I have not looked into how to do that.

[1] https://isync.sourceforge.io/mbsync.html

[2] https://github.com/JakeWharton/docker-mbsync

No API but you could script the browser. Obviously would break if the UI changes. Quick google shows some potentially still-relevant posts: https://superuser.com/q/716756

It lets you choose a bi-monthly schedule for the exports, then you just need to automate the download from the link that you get emailed.

Google takeout offers build in automation, you can set it to run every X months automatically.