I recently tackled a full degoogle. I still have my email running, I really wouldn’t recommend deleting it for legacy purposes. But the tens of thousands of emails I had are fully deleted from gmail. I unsubscribed from basically everything, and hardly get emails there anymore.

I used Google takeout to get the emails exported. I had to do this per gmail account. They give you something called a .mbox file. I now use proton mail and pay for their service (I think paying makes sense for the service given the alternative is getting your data scraped for advertising revenue). I stored a backup of the .mbox file in my proton drive, and loaded the file in thunderbird (on linux with an encrypted drive) for whenever I need to search those old emails.

Overall it’s pretty easy. The hardest part was a bit of Thunderbird jank for first time setup. This guide was helpful: howtogeek.com/…/how-to-open-an-mbox-file-in-mozil…

Good luck :)