I have the Scar 18. I just upgraded the memory from 32GB to 64GB Kingston Fury 5600 (which only runs at 5200). As far as the NVME restrictions, I'm not aware of any. There are two slots, and I believe each slot is fully capable of running a 4TB drive...
I finally got mine to work. I have the KF556S401BK2-64 set just like OP. I could not get it to boot no matter what I tried. It would turn on, but it would never get past the memory training.
I put the original 32 GB back in, and the same thing happen...
