I am using a VK-3 with ASRock X570 ITX/TB3 motherboard. The only NVMe slot is on the back of the motherboard. A M.2 drive mounted there will be sandwiched between the motherboard and the GPU riser cable with no air flow and no room for a heat sink.
If you want to use that slot (I had to since it was my only one), use an M.2 extension cable. I used
NGFF M.2 PCIe M-Key Extension Card with 20 CM Cable
www.eproductsolutions.com
NGFF M.2 PCIe M-Key Extension Card with 20 CM Cable, part number M2-1442-EXT
(search for the part number, several places sell them)
I used it and put my M.2 drive and heat sink on top of the PSU. There is also plenty of room for it above the graphics card.