If you want encryption at rest with an ssh-accessible data store (like rsync.net) then you should use borg[1][2] - the "holy grail" of backup tools.

This is unrelated to ZFS send/recv, other than the fact that we are also the only cloud storage platform to offer borg/attic support.

[1] https://www.stavros.io/posts/holy-grail-backups/

[2] http://www.rsync.net/products/attic.html

Not until OpenZFS offers encryption, which might be... in a year or so? There are signs that it's coming along well.

I don't know if it's possible with FreeNAS, but on an ordinary Debian box nothing stops you running ZFS on LUKS encrypted devices. I've been using this configuration without issues for about a year on a home file server.