ZFS pool continually DEGRADED or FAULTED

Solution 1:

After almost a month of debugging, it's safe to say that it was indeed the chipset's SATA controller.

@shodanshok brought to my attention that there is a "significant age-related SATA issue" with intel chipsets, and some extra googling showed that I wasn't the only one.

I've bought some new hardware, alongside a LSI 9205-8I H220 to connected all the drives into. Without any changes to the configuration (apart from a more modern motherboard + CPU), they ZFS pool was imported with no issue and the pool has been running for a whole day with 0 checksum/read/write errors. By now it would have been in the hundreds. This confirms that the issue was the onboard SATA controller.

Hope this helps anyone who is experiencing a similar issue!