Just to be clear, you don't encrypt the source pool and then just pull ZFS snapshots to your encrypted backup pool and want encryption there, right?
Because simply encrypting on the source would be easier otherwise.
ZFS is a server filesystem...
If you have configured email notifications, you will get SMART errors.
I am not 100% sure if you also get ZFS and scrub errors by default, but I guess that is the case.
What makes you think, that PBS sending mails to one single e-mail address would trigger an IP block on spamhaus.org?
Where? On spamhaus.org?
Did you do that?
ext4 on raid5 on a P420i with 1GB FBWC, which essentially acts as PLP for HDDs, is fine too. The cache is big enough to hold the metadata, and all the atime updates during a GC run will be caught by the cache, and eventually written in bulk.
raidZ1 is known to not be good for IOPS: https://forum.proxmox.com/threads/fabu-can-i-use-zfs-raidz-for-my-vms.159923/ . And your drives might be consumer drives without =1&c[nodes][0]=16&o=date']PLP. Lots of threads about disappointing (write)...
Thanks everyone for helping me with this problem.
I decided to move all data to another pool (no SMR drive) and remove those drives from the server.
I will mark this as Solved.