Just an update. The 'Wrong or no file system' issue came back every time I shut down my Win10 VM. Time for a cache reformat!
I decided to Move everything off the cache using the Cache:Yes and initiate Mover to get it on to the array. Some stuff got a bit stuck but I used Dynamix File Manager to shift it across on to the array manually. I changed the file system to xfs on the cache drive, formatted it, then changed back to btrfs and reformatted. I set the Shares I had on the cache back to Prefer and moved everything back on to the cache. Whilst it seems like that should just work, I had all sorts of issues with my docker containers and VMs that didn't appear in the tabs as they couldn't read the appdata properly. I had to restore my original appdata from backup, then go in to Apps and Previous Apps to download my containers again from the saved templates and set to read from the restored appdata. Once I did that, the dockers came back with their previous settings. Also after that, the VMs appeared and were able to boot with no issues. Another issue that I had with the docker container was that the Plex database was corrupt. I tried SpaceInvaderOne's solution on his very helpful YouTube video, but that didn't work either. I did have 2 or 3 database backups in the same folder and replaced the database with the last backup. That's now working.
A bit of a learning curve for me, but I hope that I have everything fixed now and a more stable cache drive!