You are correct, in general, NVMe solid state drives are 4 to 5 times faster than SATA solid state drives.  Simply pointing out your system storage capability.  It is possible to add a second NVMe drive using PCIe x4 slot.  Adding SATA SSD solution can free up M.2 slot for larger capacity NVMe as your game drive.  You can use SATA SSD for operating system and apps. 
My G5 5000 has a 500GB NVMe mounted on the motherboard in an M.2 slot.  I also have a 2TB Hard Drive.  I am running out of space on the 500GB which holds my OS (W11), and mainly MSFT Flight Sim.  I would like to keep my Flight Sim on an NVMe drive for speed of loading, as well as the OS of course. I thought I would look for an expansion card that would hold a couple of M.2 NVMe cards, one for the new 1TB or 2TB NVMe I'm planning to buy, and one for future, but the expansion card slot (Slot Designation SLOT4) is a shorter one, even though it is called a PCI-E Gen 2 x16, so I cannot use an expansion card with the full connector like the ASUS that holds four NVMe M.2 cards.  Almost all expansion cards I can find with two card mounts appear to be for one NVMe and one SATA card, which leads me to believe there is a limitation on the number of NVMe cards my motherboard can handle.  I did find a card that has two but says the motherboard & bios must have bifurcation support.  Can I even add an expansion card with a second NVMe, or should I just put the new/larger one directly on the motherboard, and let the old 500GB go un-used? Would the NVMe mounted on the motherboard be noticeably faster than one mounted on the expansion card? Any other suggestions? [G5 5000 1710700F CPU @ 2.90GHz, 16GB memory, AMD Radeon RX 5700 XT graphics w 8GB memory]
Adding a second NVMe M.2 to my G5 5000