I don´t use the NVMEs as a Cache.
But follow the Numbers on my Screenshoot, I´m not sure but you will find it than.
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If you use SATA SSDs, a cache is useless anyway.
Even with built-in HDDs, the cache doesn't really do much.
The cache only becomes really effective when lots of people are constantly accessing the same files together.
It makes much more sense to use NVMEs as volumes. Install all apps, Docker, and VMM on the NVME volume.
Install as much RAM as possible as cache. Linux likes to use RAM as cache. 
I recommend Kingston RAM, Have a Look to the Compatibility List for Kingston on DXP4800+, or have a look to my Signature (under the Post -> Spoiler) That one works nearly for one Year perfect, had never Trouble with it.
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