Update: Moving the SSD was not a sufficient fix in this case. After adding another hdd to use as cache for my file ingest activities (until the paired ones arrive), I had three shares which still used the SSD for their cache pool - appdata, system, and one I missed somehow when I moved the rest.
Long story short - I backed up an external system to that share. I wrote 500G of data to the drive at gigabit transfer speeds and it reached 53C! I completely filled the cache drive, which probably contributed to corrupting the docker image. Fortunately, the procedure for fixing the latter is straightforward. Comparing the data to the original after it was moved to the array was also successful, so it was mainly a nuisance.
I’ve now confirmed that the only active thing on that drive IS appdata (system is there, but I’m not using it). All caching is going to a spinning disk. I’ll replace the consumer-grade ssd with a U.2 SSD once the shipment arrives, as well as replacing the single disk cache with a mirrored pair.
I don’t know whether moving the SSD to a more actively-cooled location would help. That location is not good for this drive seeing that much write activity - but then again the drive probably can’t survive that much activity for long anyhow, so I’m changing the usage pattern instead.