ASRock X370 Pro - Can't POST with 8 GPUs

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   I am currently transforming my gaming PC into a mining rig by adding PCIe slots and GPUs, hence the X370 (unusual choice for mining)
The GPUs are 8xASUS RX 580 Dual 8GB

I get "0d" error on Dr. Debug and cannot even POST (blank screen, cannot enter BIOS) when I install 8 GPUs

Everything works fine with 7 GPUs (I can remove any one GPU) everything works fine.

However when adding 8th GPU, it can't POST and always displays "0d"

I have tried replacing 1-2 GPUs with 2 spare GTX 1050Ti, neither combination works.

Mix and matching my 10 GPUs works fine if there are only 7 GPUs total. Adding the 8th one, whichever it is, always results in no-POST.

I am using some kind of PCIe PLX card (not sure what to call it) to turn a single 16x PCIe to eight 1x PCIe (see pics below)

I have tried using combination of the PLX card and other PCIe slots to no avail (e.g.: 6 GPUs on PLX card, 2 GPUs on mobo's PCIe 1x slots)

I even have a second PLX card and have tried swapping them or even using them together (e.g.: 4 GPUs on the first PLX card, and the rest on the second PLX card)

Neither works if I install more than 7 GPUs
Again, everything works OK if I only use 7 GPUs max, regardless of which PLX card or PCIe slot I use.

I have tried replacing the PSU (I have two SilverStone 1500W and 1300W Ti), replacing PCIe riser (I have 20 of them), replacing the PCIe cables (total 16 PCIe 8pin cables from both PSUs), replacing PLX card (I have 2 of them), all to no avail.

I have started to suspect that I have either hit the processor or the mobo limit (Ryzen mining mobo from Biostar only supports 6 GPUs)

Is there a maximum GPU limitation on AMD Ryzen itself?

Or is this limitation on ASRock mobo?

Could someone experienced enlighten me?

Any help is appreciated!

My specs and pictures are below

Mobo: ASRock X370 Pro (BIOS v4.40)
CPU: Ryzen 1700
GPUs: 8xASUS RX 580 Dual 8GB, 2xGTX 1050Ti 4GB
RAM: 2x8GB Corsair Vengeance RGB DDR4 PC3000
SSD: WD Blue 1TB (M2 SATA)
PSU: SilverStone Strider 1500W Ti and 1300W Ti (using only one of them)
OS: Windows 10 Pro (should not be an issue since the system cannot even POST)

PLX 1->8 PCIE slot extender (see pics below) bought from Amazon
No brand PCIe 1x risers bought from Amazon

Pic of whole setup:

Pic of Dr. Debug error code "0d" when using 8 GPUs

The PLX card that I'm using to create extra PCIe x1 slots:

P.S.: After failing to POST with 8 GPUs, and I remove 1 of the GPU, the mobo seems to "remember" and I cannot POST unless I clear the CMOS
Clearing CMOS has no effect when 8 GPUs are installed

Will provide more information if requested