Why are you trying to cache so much?
I always recommend NOT caching the initial data load. Cache and Mover just get in the way since cache won't have the capacity. There is no way Mover can move to the slower array as fast as you can write to the faster cache. And if you are trying to do both at once they will just be competing for the disks. Mover is intended for idle time.
Many of us including me don't even bother to cache user share writes. Most of my writes are from scheduled backups or queued downloads, so I am not waiting for them to complete anyway. I only use cache for my dockers (no VMs for me), for temporary storage for DVR performance, and for copies of some frequently accessed files so my array won't have to spin for those.
Unraid is linux, and linux will use free RAM for I/O buffering. All that space on the graph described as "Cached" in the legend, between "Used" and "Free" is just that I/O buffering, and completely normal.