There's still a fair amount of writes happening at any given time to either the docker.img or the appdata, so your parity drive will stay spun up all the time.
Read speeds will increase, but writes will still be slow (and may get slower as array devices do not support TRIM)
So far as redundancy is concerned, most people with only a non-redundant cache pool will simply back up the appdata on a schedule (weekly?) via the appdata backup plugin.
IMO, your best bet is to use the SSD as a cache drive.