Out of Slots on motherboard What are my options?

Why the 4x NVMe? Why not a mix of NVMe + SATA III SSD? Add in cards to go in an empty PCIe slot 1 or 2 drives in one of

NVMe SSD 50% slower on socket 1

nvme 40 in 30 slot Only if its a boot device or your intent is software raid, any most pcie adapter regardless of bios support will support an nvme data drive once

nvme and m2 slot difference 2 NVMe slots These are the very fast interfaces for your SSDs The 'x16' on the expansion slots is the number of PCIe channels on