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.
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.