Well, I just learned that PCIe M.2 slots cannot be disabled in BIOS. Makes sense I guess.
So, I had to tear it down, remove all NVMe drives except the one I wanted to install windows on... Put the rest of the parts back together... install Windows 10... tear it down again... reinstall the other NVMe drives...put it back together.
needless to say, that was a monumental PitB :-/
Anyway,
@Going Bald was right.(Thanks for the nudge) Getting those other drives out of the way was the only way to install Win10 in this case. When will MS decide to become multiboot aware?