HIDDEN FUNCTION!? SOLUTION which boots M.2 drive and shows all 3 drives on F12 Menu!
So, as I'd written above, I had found that by setting both SATA drives to "Disable" in the Boot Order, that would always show the M.2 NVMe drive in the F12 Boot Menu and boot from it; unless I clicked on one of the two 'empty slots' above its name... which would then boot up from a SATA drive.
Well, today, I finally decided to click on something I hadn't before, BECAUSE what the instructions say about it, do not imply at all what it actually did:
"Boot Override
Allows you to select a device to boot immediately. Press <Enter> on the device you select and select Yes to confirm. Your system will restart automatically and boot from that device."
First picture attached shows that screen, and after doing so, it not only booted into the M.2 NVMe drive, but after shutdown and then pressing F12 Key next time, it now shows the M.2 NVMe drive in what I believe is the First boot drive position! (With the two SATA drives appearing above it... Second attached picture. So now, all 3 drives appear at the same time, and ever since I clicked on the M.2 NVMe drive in that F12 Menu, it is still booting into only it.
So, that's why I believe I had to choose the M.2 MVMe drive in the "Boot Override" Menu.... Because if I did not do that, it would boot from one of the SATA drives, and never show the M.2 NVMe drive in the F12 Boot Menu. Again, I still believe this kind of crazy way of having to do this was because either: 1) GIGABYTE and/or AMI BIOS really messed up by giving SATA drives the boot priority over the M.2 NVMe drive, and/or 2) There's some 'flaky' messed up code in the BIOS.
Dan, TheStarman.