I made the mistake of not setting up encryption on my main 45TB zfs pool so I’m currently backing up everything on there to tape so I can recreate the pool (also need to change from mirrored to raidz) and then copying everything back to the drives. Although writing and reading each are around 6 days continuesly. Didn’t want to bite the bullet and pay more then I absolutely had to and only got a LTO-4 drive and tapes.

Wasn’t raidz expansion already in a tagged version or did I miss remember?

If you’re talking multiple Terrabytes and are located in the EU you might want to consider AWS Glacier I have like 6Tb on there and pay sub 20€ p.m. If you’re in the EU you can request one free migration download by contacting the support. Otherwise you’ll pay thousands.