Do you expect to need all that capacity soon? I always say each additional disk is an additional point of failure. With so many data disks I definitely recommend a 2nd parity disk.
Depends on how you use cache. You already have appdata on cache, that will be the largest of the 3 "cache-prefer" shares. system share shouldn't take much space, docker and libvirt images are set to 20G and 1G respectively and that is all that should be needed. domains share depends on how much space you need for your VM OS vdisks.
The other shares shouldn't keep things on cache, and you might even consider not caching some of them. If you have large data transfers to do, cache can just get in the way since those have to be moved to the array eventually, and mover works better during idle time. You can't move to the slower array as fast as you can write to the faster cache, so for large transfers, don't cache.
With 224G cache, I think that should be enough unless you expect to write more than 200G per day on a continual basis, and if you do, you can just not cache some of it. Just consider when the faster cache writes are really beneficial and only cache those shares. For example, I have a share for backups of my PC. Those are scheduled unattended processes and I don't care how fast they get written since I am asleep, so they go directly to the array.