Think I was seeing this same thing on a smaller Adata SSD the SU635 / 240GB. MakeMKV docker pinned to 1 core writing just 30MB/s to cache SSD would get all cores/threads to a high iowait after 15 minutes or so. Found this thread and swapped the cache to XFS and the iowait impact is much reduced now.
Reads from the SSD to array were 200-300MB/s no problem when I was moving stuff off cache so I could reformat, no iowait at all.
Looking forward to update, but if you don't need a cache pool I guess just sticking with XFS isn't a bad solution seems to work well this way.