Tutanota was the one service I found that let you add (IIRC) arbitrary amounts of storage to your plan, but I found their web interface to be kinda clunky, and let the account expire. I keep wanting to look into this again, but never get around to it.
There's no official Linux client, but there are open source ones that perform well.
You could drop your mbox onto a OneDrive sync and then use fetchmail or similar to rip it out of your provider. Each email could be individually encrypted, so you won't have to deal with a 500gb blob to sync.
This is what it took:
* Connect to Outlook and sync locally
* Sort All Mail by sender
* Delete All Mail per senders that aren't important
It took me a day, and I'm glad I did it. Not that painful. Outlook was a champ through the whole process.
Now I can move somewhere else, which I will.
I remember services available 5+ years ago that would help you with this kind of thing (scanning through email archives by sender), but the ones I remember have all morphed into "tame your inbox" apps, which doesn't solve the same problem.
I still use a Google account for my Android phone, but my PinePhone should be shipping in a few weeks.
I'm done with Google. Good riddance.
With a paid account it is trivial to export your emails as well: https://protonmail.com/support/knowledge-base/export-import-...
My gold standard is mutt with its powerful and fast search filters (gmail is a serious downgrade).
Protonmail's lack of search is crippling.
Email export ... I don't even want to think about.
It's fairly easy to extract your data.
I use Outlook to access it on my work PC, the Android app on my phone, and the web interface elsewhere. Seems quite usable and painless, to me; even search.
But then, I fastidiously organize my email and so I don't need to rely on "smart" , privacy violating, deeply intrusive search.
You have no effective recourse should Google Googleify your Gmail account.