Proton Drive does not have an official API which might be a dealbreaker if you're looking to use rclone for file transfer. People have been clamoring for one for years and I think they are finally taking it seriously now.

OP did not mention how much storage is required. Most consumer cloud storage will give you a few GBs on a paid plan, maybe 100GB or something like that. I think it would be better to rent a storage server and install Nextcloud on that in order to have a much larger capacity, with the added benefit of being more private (specifically if you're using full disk encryption).