Ok - so I purchased a third 1TB 980 Pro and installed it in my SSD7103. Now two 980 Pros are visible in Disk Utility. Noted the S/Ns of the visible NVMes, and opened up my SSD7103 to find it was the 980 Pro in the top slot that was not visible. Also note that I had tried swapping the ones in slots 1 and 2 already, so apparently there was a problem with slot 1 (top) and not with that 980 Pro. I moved that 980 Pro to the bottom slot (4) and booted to find all 3 visible.
I've asked HighPoint support for advice, and if they have RAID drivers that are stable for us yet. FWIW - I found a YouTube video where a guy says he installed 4 NVMes in his SSD7103, and only 'saw' 3 of them until he got updated firmware from HighPoint. The plot thickens...