Can only get to BIOS if I unplug ALL of my graphics cards.. Even having inserted one of them during boot results in black screen.

Here is full list of hardware:
MOBO: Z170A Gaming Pro Carbon (MS-7A12)
        BIOS: E7A12IMS v1.80
CPU: Intel(R) Pentium(R) CPU G4400 @ 3.30Hz
GPU: 7 x RX470 4gb
RAM: 2 x 4GB Corsair DDR4 SDRAM

Boot order:
1. USB Key
2. UEFI USB Key
3. UEFI Hard Disk
...

Have the same specified advanced settings as instructed on simplemining installation guide.