Sharing my experience as a user that was also facing this SMART read issue but found a solution, for what it's worth;
The culprit was my "system" share was set to Cache-Prefer but my docker.img file was still sitting on my array, meaning any reads/writes were causing a spin up. I guess Unraid then took the opportunity to read SMART data as the drive was active which is why I saw it in the logs.
Note, I do have mover scheduled but evidently it couldn't move the docker.img file because Docker was always enabled and the file in-use. I assume a file that is in-use is prevented from moving, I don't know that for sure. After disabling docker, running mover, seeing the 40GB move from my array to my cache drive, and re-enabling docker, the spin-ups and SMART reads have gone away.
Also, the heavy hard drive activity I experienced when enabling or disabling docker no longer occurs, presumably because it's no longer hitting the docker.img file on my array. If anyone else sees that symptom, perhaps my solution with work for you.