his MoBo has a “VGA-SWITCH CARD” in it. Label says “something” must be in that slot. I’m not seeing anything about it in the manual I downloaded.
See page 2-20. The page says if you want full PCIe x16 speed you need to insert it, otherwise the GPU will use only PCIe x8.
Don't worry about any of that right now. Just see if you can boot to the OS without the graphics card.
Noted, however, trying with the graphics card pulled was in my OP (thread starter). No go.
Next I think is pulling everything possible not essential to booting, and trying to boot, and, checking actual power supply voltages.
Note: When powering up, Caps Lock and Scroll Lock on the keyboard both flash, then go out. A bit later, Num Lock comes on and stays on until I hit the key to turn it off. Green pwr LED on MoBo lights and stays lit. Mem test LED checks ok.