This. It’s not as simple to get it working as it is on non-free OS’s, but with rclone I can get on Linux pretty much the same functionality I get from (eg.) Google Drive on Windows, including have most of the drive with on-demand access (meaning files are not stored locally, but downloaded / uploaded as needed) with a few specific folders synced for offline use. Since it supports a lot of storage services, I suppose it shouldn’t be that different to set it up the same way with Proton Drive.