Reddit mentions of Supermicro AOC-SLG3-2M2 PCIe Add-On Card for up to Two NVMe SSDs

You should check out this it's only 8x, so two m.2 slots but it's cheap enough to buy two. I picked mine up for $40 on Amazon.

I have that board and can confirm the option exists in the bios. I am considering this card to try for a few reasons... it's 8x form factor so it'll fit the motherboard. I'm using a 847 chassis so can only fit half height cards.

Yeah, that's the one distinct disadvantage of the consumer AM4/Z390 style platforms. They offer enough lanes for most typical users, but when you start tacking on a lot of I/O, GPUs or networking they quickly find themselves staring at a PCI Express lane wall. Platforms like X399/TRX40/X299 offer more lanes, but come with their own set of disadvantages (primarily price).

I did a little bit of looking around and could not find any information indicating that your specific board supports lane bifurcation. It's possible that there's a modified BIOS floating around which would enable it, but I didn't see anything on the sites I checked. My thinking was that if it were supported, you might be able to utilize a 2xNVMe M.2 -> x8 PCI Express card such as this ~$50 one from Supermicro, which would leave you with a fairly adequate x8 configuration on your GPU as well. The downside is that it's "only" two ports and requires that bifurcation support to operate. You could also plug a generic $15-20 single NVMe M.2 -> PCI Express x4 adapter into that second slot, for use with a single drive. It'd be at full speed, and your GPU would again operate at the lower x8 link state.

It's still worth exploring the use of additional SATA SSDs on any remaining open ports you may have - they can be configured in RAID 0 through the BIOS or through Windows itself.

Yeah, I have 32GB ECC RDIMMs as well - I got it for $70-75/stick and have seen similar prices.

This is probably the AOC "HBA" too: https://www.amazon.com/Supermicro-AOC-SLG3-2M2-PCIe-Add-Card/dp/B071S3ZY8P#customerReviews

I did some research and apparently that's not universally true. If you have a motherboard that supports bifurcation on the pcie slot, you can use a much much cheaper adaptor like this one https://www.amazon.com/Supermicro-AOC-SLG3-2M2-PCIe-Add-Card/dp/B071S3ZY8P

Now I just need to figure out if my motherboard supports bifurcation... Not sure where to find that...

Found it: https://www.amazon.com/dp/B071S3ZY8P?tag=tinkertryamzn-20