I ended up using a similar command to what
@ChrisRJ posted on my drives and the problem appears fixed. Almost 24 hours later the load cycle count hasn't increased at all. Is there any reason you use the -s option if you run the command on boot?
When I finally got around to sharing a dataset and copying a significant amount of data I had a couple of the drives fault in response to persistent write errors. The logs are full of
CAM status: Uncorrectable parity/CRC error
entries for almost all drives. I'm in the process of running a SMART long test on all drives - you're right, it estimates it'll take around 12 hours. SMART reports a high
UDMA_CRC_Error_Count
for a lot of the drives so I'm hoping it's just bad SATA cables. I've ordered some replacements but at this time of year they're probably going to be sitting in the post for a while.