I would suggest removing one of the devices from the pool to test, if the same add it back to the pool, remove the other and test again, just to check if it's one of them slowing down the pool.
Since it only happens with that Seagate model in either controller it suggests they are not liking something in the newer kernel, look for a firmware update for the drives, failing that you'll probably will need to wait for some kind of fix in a future kernel.
Yes, sorry, both disks look healthy, with the array stopped, unassign both, start the array, stop the array, then see if both unassigned disks mount with the UD plugin, if yes check that contents look OK.
Backup current /config folder and recreate the flash drive using the USB tool (with v6.11.1), if OK restore the config folder, if it also fails replace the flash drive.
This suggests a problem with the pool devices or the pool itself, is the pool old? Some time ago partitions on SSD devices were changed to be 1MiB aligned for better performance, also are you regularly trimming the pool?
Also note that and an LSI won't improve this, disk to disk transfers inside the array are always slow because of how parity works, pool to array transfers can be helped if you enable turbo write.
9211-8i is PCIe 2.0, 9207-8i and 9300-8i are PCIe 3.0, note that for up to 8 HDDs they will all perform the same, avoid buying from China, bigger chance you'll get a fake, look for used server pulls.
Nothing in the log that I can see, do you hear the drive spinning up? You can also install a BIOS on the HBA then check if it's being detected there, if not it also won't be detected by Unraid.