I started out with davfs2 but it was a) very slow at uploading for some reason, b) there was no way to explicitly sync it so I had to either wait a minute for some internal timer to trigger the sync or to unmount it, and c) it implements writes by writing to a cache directory in /var/cache, which was just a redundant 10GB copy of the data I already have.
I use `rclone`. Currently rclone doesn't support the SHA1 checksums that Fastmail Files implements. I have a PR for that: https://github.com/rclone/rclone/pull/6839