Quite a lot of writes to SSD cache, how identify? (looks like writes to docker.img)

[Squid]

Switch to a docker "folder" instead of an image.  By definition it will give the least amount of writes since there's no amplification created by having an image

 

There is nothing "Unraid Specific" about docker.  It's the same implementation as on any other distribution.  Your issue with Plex et al is quite possibly health checks which can easily be disabled via an extra parameter

--no-healthcheck