I think on recent kernels, using the hipri option doesn't get you interrupt-free polled IO unless you've configured the nvme driver to allocate some queues specifically for polled IO. Since these Samsung drives support 128 queues and you're only using a 16C/32T processor, you have more than enough for each drive to have one poll queue and one regular IO queue allocated to each (virtual) CPU core.
It's terribly documented :(. You need to set the nvme.poll_queues to the number of queues you want, before the disks are attached. I.e. either at boot, or you need to set the parameter and then cause the NVMe to be rescanned (you can do that in sysfs, but I can't immediately recall the steps with high confidence).
