This is weird. I changed appdata from cache: prefer to cache: yes, then invoked the mover. It's been running close to an hour so far.
As you can see in the pictures, my usage has returned to normal in the first image. The second is my shares with usage broken down by drive. It transferred 14.6 GB from Cache to Disk 8, and managed to drop used from 859GB to 308GB.
Could it be something to do with sector size on the cache? My plex metadata is in appdata, so far it has transferred 18621 files in the plex folder share, totalling 7.2g.
14.6G total across 36265 files. Maybe they are taking up lots of space despite being small?
Then again, probably not. The cache drive portion of appdata still has 539131 files. I really don't understand what caused this problem in the first place. If moving 14G cleared up 500G of space, something just isn't right.
EDIT: The pool drives are GPT 4kb-aligned. The cache is MBR: 1MiB-aligned.