There is no need for custom Bios , P1.20 is fine.
If you use powered risers do not use 4 pin Molex onboard.
You can use the iGPU or not up to you, i'm running 5 and 6 cards + igpu with this board.

Disable Render stand by in mainboard bios.
Disable sound in mainboard bios.

Is your Chipset driver installed?
Remove GPU drivers in safe mode with DDU.
All cards are connected through risers? Or do you plug in one card in PCIe 16x?