Just FYI: You can adapt the M.2_1 slot to a regular PCIe slot and use non-storage/non-NVMe PCIe devices with it.
I’ve done many PCIe experiments but ASUS X570 motherboards are a bit iffy here since their UEFIs don’t show the PCIe Advanced Error Reporting (AER) and it’s disabled by default. 
I currently experiment with ASRock X570 motherboards, they can enable PCIe AER however it is only actually enabled for PCIe slots handled directly by the CPU, with chipset slots you’re basically “flying blind”.