Today I've seen a picture of this motherboard(Chinese source):
We can see an AM4 socket but no chipset, so something like A300/X300, PCH-less design.
The GPU onboard looks like a navi10(RX 5700 Series), and the CPU/GPU mounting is kind of weird, no mounting holes near GPU at all, and only 2...
It's a Silverstone SX650-G. I did not do anything else. The psu appears to be a slightly off from the top frame, but you can mount it with fewer screws, like 3 or 2, as some of the wholes are still aligned.
But this is not always the case. Rome servers are coming and there ARE PCIe4 bases systems. Risers/NVMe cables are widely used in those servers. We may see those pcie accessories coming soon, but don't expect them to be cheap. There are some 100Gbe dual port NICs already based on PCIe Gen4.
I'm in the same situation as yours. Just mount the psu with the screw that will fit in the whole. and you will be fine. Use the port that is further to the top of the case will make it easier to mount the psu.
You have to prioritize the most important aspect to you, small? cool? or quiet? Because for most of the time, you cannot get all of them when you are using powerful components.
Small and Cool usually turns out to be a very loud configuration(very high rpm fan in action); Cool and quiet usually...
I will probably go for the Asrock offering, the Thunderbolt is quite useful to me, at least I could add 10GbE to it. I think such high speed external bus is one of the most optimal usage of the PCIe lanes for as sff PC.
Open air GPUs usually do not perform better in such a small case. Due to the limited airflow in the case, the exhaust of those cards cannot be removed from the case fast enough and the ambient temperature in the case will go up because of that. Blowers push exhaust directly out of the case.