The differences aren't as big as they seem ...
I am in the planning stages of deploying Proxmox to a 2tb|2tb + 3tb|3tb zfs array and after a bunch of reading, I understand that zfs recordsize and qcow2 cluster_size shoud match each other exactly.
With version 4.2 of the Proxmox Backup Server, pull/push sync jobs can be encrypted and decrypted server-side and processed ...