You can ZFS send proxmox backups to rsync.net over SSH

Just a note - 'zfs send' capable rsync.net accounts have a 10TB minimum size (because we need to run a VM for you and give you a real ipv4 address, etc.)

However, we're pushovers and we keep letting people in for 2TB or larger. Still a minimum, however, and note that "normal" rsync.net accounts cannot be accessed with 'zfs send' (although they are still on a zfs filesystem, of course).