So next I’d be checking logs for sata errors, pcie errors and zfs kernel module errors. Anything that could shed light on what’s happening. If the system is locking up could it be some other part of the server with a hardware error, bad ram, out of memory, bad or full boot disk, etc.

The drives in the zpool, are they SMR drives? Slow write speed and disks dropping out are a symptom of I remember correctly