ASRock Rack prepared their new crazy chipset-less board. Now in mATX form-factor: https://www.asrockrack.com/general/productdetail.asp?Model=EPYC4000D4U
Also noticed chipset is listed as KNOLL3 X600 rather than X300. Probably this is why it’s more flexible than their AM5 deep mini-ITX boards. You can use PCIe x16 slot in x8 mode, populate 2 M.2 slots and still get MCIO1 with full x8 lines.
All of them will be raw CPU PCIe 5.0 lines so no any problems with passthough.