Not OP, but I have a Google Apps Script running on daily schedule which extracts every email older than 30 days as .eml file, renames it as YYYY MM DD HH MM SS SUBJECT.eml, saves in a Google Drive Folder with YYYY as subfolder. Drive itself is synced instantly with my hard drive, and I backup that drive occasionally (like around 3 to 5 weeks) to another local & rclone to Dropbox.

30 days so that I can have enough time to delete it. Trash & Spam is excluded, and every successful downloaded email is labeled to prevent it getting downloaded again in next run.

Originally 4 years ago when I started this script, it was timing out because of about 15 years of emails. I ran it for 6 minutes only, every hour. Now once it caught up, it needs only few seconds, and there are only like 5 to 20 emails caught in its daily net.