A client that works really well with SFTP is lftp. It is in most linux distros and is available for mac in brew and in cygwin for windows. It has a mirror subsystem that works nearly like rsync and can support SFTP + chroot. Combine that with rsnapshot for diffs and you can fall back to earlier versions without using a lot of disk space. You can put that rsnapshot location outside of the SFTP chroot so that malicious access can't wreck your data, or just make the directory above your rsnapshots root-only access so regular users in a non chroot environment can't get there.

This is also the direction I'm taking. It makes everything just easier when everything is file-driven, backups are simpler, and when nothing works anymore the basics always work.