Desktop mainboard for VFIO build (software development and test) with ECC memory support

I am looking for a new mainboard for multiplatform software development (Linux/Windows) with authentic GPU behavior (GPU passthrough). After some research I have found out that an AMD Ryzen 5900x 12-core CPU would be suitable for this task. I already have 64 GB of DDR4 unbuffered ECC RAM (2x32GB), two 2TB PCIe 3.0x4 SSDs, and a PCIe 1.0x1 sound card (not much bandwidth, but a card anyway).

Thunderbolt 4 is planned to add someday, to connect some low latency audio equipment later on. The Asus Xonar Xense sound card which I have is quite old, but with a superb DAC/ADC for headphone/mic. I use dbx 286s mic preamp at the moment with a Rode Procaster dynamic mic, so XLR input is not necessary. Audio is not the main focus, but I want to have a clear voice when communicating, and drive the Sennheiser HD 660S headphone as well. I have two old electronic musical instruments I want to connect someday in the future which would mean a multichannel audio interface. Furthermore, I have an old 5.1 receiver not supporting HDMI, which actually is connected via S/PDIF Out. There is a plan to replace it someday by a more modern and energy efficient model.

For graphics, I have to get a pair of dGPUs, not a pleasure at the moment. My plan is to buy a GT 1030 and a more capable dGPU, albeit used, for a start. The mentioned CPU does not have an iGPU, and changing it to a 65W 8-core APU (5700G) just to have one does not seem appropriate for the main task mentioned.

People recommend the X570 chipset over B550 for IOMMU flexibility. So I have boiled it down to two options:

Both mainboards have two slots for graphics cards (x8/x8) which is good.

The Asus board has Thunderbolt 4, 10 Gbit/s Ethernet, and two Ethernet ports. I do not need anything of it at the moment, but this seems to be a board with all features I would think of adding later on. I can even insert my sound card without stopping Thunderbolt operation, because of added bandwidth coming from the X570 chipset. If I fill the M2B slot, the second PCIe graphics card will run with x4, just enough for a GT 1030. But the board has three M.2 slots, so this is not a problem, just populate M2A and M2C slots.

What I dislike about the Asus board is the experience people have with IOMMU (B550 variant), with too many components residing in one single IOMMU group. I do not know if the X570 variant may suffer from the same issue. Furthermore, you cannot choose the Initial Display Output in BIOS, at least in the past, and the manual does not mention BIOS settings to review before buying. And Asus strangely dropped S/PDIF optical output for the X570 model only. As long as I can use my sound card, this is no problem, but it reminds me of the smartphone market where premium products get features removed (like the 3.5mm jack). Yes I know that a USB S/PDIF adapter is available for cheap, but I wonder about the benefit of such a move.

The Gigabyte board has a much better VRM, five M.2 slots for SSD (lacking I/O to support it though), and a history of virtualization support. I do not believe that I will need more than two M.2 slots ever, but an efficient VRM is a plus.

What I dislike about the Gigabyte board, and really took me by surprise, was the lack of PCIe slots. Five M.2 slots without proper lanes, and only three PCIe slots. I cannot use the sound card and Thunderbolt 4 addin card at the same time, no space. Of course, a Thunderbolt audio interface like the PreSonus Quantum 2626 could replace the sound card, and if I want to use S/PDIF Out, the mainboard provides it.

I thought that maybe the Gigabyte X570S Aorus Pro might have more than three PCIe slots, but no, it is the same design.

ASRock does not seem to want to compete in this market segment, and MSI has not heard about Thunderbolt or ECC RAM support (below HEDT).

AMD plans to offer a 12-core Threadripper Pro with 8 memory channels. I assume that for software development the memory interleave would be overkill, although it has plenty of I/O - no compromise necessary. I wonder which use case would fit for it.

Intel does a strong market segmentation. So with Z690 again I would not get ECC memory support, and low-end HEDT will come much later.