I did that once as part of an escape from Google. Just downloaded everything to thunderbird and then severed the link. Then thunderbird became my source of truth for emails. Whatever I needed to delete, I did (subscriptions, ham emails, et) and then I uploaded to another vendor which was much cheaper for the starting tier. I can skip that last step.

Just make sure to severe the link so thunderbird doesn’t do anything stupid. But you can also take a backup of your inbox in case thunderbird does do something silly.