So there's a lot to unpack here.
First, the Ryzen 3000 series CPUs (all of them, including the Zen+ 3400g) have 20 lanes + 4 lanes for the chipset. APUs use 8 of those lanes for the integrated GPU, so instead of 16+4+4, you get 8+4+4. This is the problem you are having, the card you have uses PCIe Bifurcation, which means it relies on the motherboard to split the 16 lanes into 4 sets of 4. But with the 3400G you don't have 16 lanes coming off the CPU, you have 8, so you'd only be able to run two SSDs on that card.
Now the good news is, the X570 chipset also some PCIe lanes off it. If you had a board with multiple slots, you could run that card off another slot. But now the bad news: The x570 doesn't support a second 16x slot at full speed. Here's the block diagram:
[MEDIA=imgur]a/b7tntpZ[/MEDIA]
This means that at best, your X570 board would have an 8x slot coming off the chipset. Most motherboards only have an x4 slot. The gentleman you linked was running that card in the 16x slot off the CPU, and then must have been running some low-power GPU in the x4 slot.
So where should you go from here? If this is a machine where you don't care about graphics performance at all, you can go mATX or ATX and use your card in the 16x slot and then add a low-power GPU like a GT 1030 in an x4 slot. But this requires you to change form factors. You also need to ditch the 3400g for a Zen 2 Matisse CPU (not Picasso, like your 3400G).
So if you're going to have to replace your motherboard and CPU anyway, I'd suggest getting an ITX Intel board with a cheap Intel CPU that has an iGPU and an ITX board that supports 4x4 PCIe Bifurcation, of which there are several. Intel's GPU doesn't eat up PCIe lanes like AMD's APUs.
