Need assistance in getting my 4 AMD GPU setup running on a Z10PA-U8 mainboard

I'm trying to run 4x AMD RX 6900 XT and an NVIDIA GT 710 (x8) display card on an ASUS Z10PA-U8 mainboard and Ubuntu Server 24.04. But so far this doesn't work so well.

I have an old GA-Z77-DS3H mainboard which has 1 PCIe x16 (Gen3), 2 PCIe x1 (Gen2) and 1 PCIe x16 (Gen2) slots. All 4 GPUs work on that board with the Delock x16->x16 and x16->x8 riser cables I have. So I know the GPUs are fine. But I'm only getting ~7 t/s output with a 40B model. I was hoping that switching to a mainboard that supports 4 x8 Gen3 slots would result in a good performance boost.

The ASUS Z10PA-U8 mainboard has the following PCI(e) slots setup:

The idea was to put the AMD GPUs in slot #6, #5, #4, #3 and the display card in slot #1. But with this setup only the display card in slot #1 and the AMD GPUs in slot #6 and slot #4 are recognized. Also slot #4 is producing a lot of AER error messages during Ubuntu booting. I was able to get rid of these error messages by forcing slot #4 to run on Gen2 using setpci.

But the AMD GPUs in slot #5 and slot #3 are not recognized at all. The system doesn't see them, they don't show up in lspci | grep AMD, only the two AMD GPUs in slot #6 and #4.

However, I can get 3 AMD GPUs running in slot #1 (PCIe x8 Gen2), slot #4 (PCIe x16 Gen3 forced to Gen2) and slot #6 (PCIe x16 Gen3) while having the display card in slot #5 or slot #3. The same 40B model generates ~30 t/s.

So I haven't found a solution to get all 4 AMD GPUs running on this board. Weird thing is that the x8 NVIDIA GT 710 works fine in slot #3 and slot #5. So the slots are working, they just don't want to communicate with any of my x16->x16 or x16->x8 riser cables. I've already tried different riser cables and also a riser card, with the same results:

• slot #1 -> any riser card / any riser cable -> any RX / GT 710 = works
• slot #3/#5 -> riser cable -> RX = not recognized
• slot #3/#5 -> riser card -> RX = not recognized
• slot #3/#5 -> riser cable -> GT 710 = works
• slot #3/#5 -> riser card -> GT 710 = works
• slot #4/#6 -> quality x16->x16 riser cable -> any RX = works

Unfortunately it seems the BIOS of the Z10PA-U8 mainboard doesn't offer the option to configure the PCIe slots. I've already got the latest version.

Does anyone have any idea what I can try or can give me a (budget) mainboard suggestion that supports 4 GPUs at least at x8 Gen3?