Ah. That makes sense. Thanks for the explanation!

Speaking of M.2, I did some digging and it looks like I was incorrect about the WiFi card and being able to access/remove it. It seems like it can be user accessed, though not designed to be.

https://www.reddit.com/r/hackintosh/comments/8eew1y/hz370n_wifi_card_replacement/dxv9xv9

https://i.imgur.com/qxuEr1E.jpg

I don't need/use the WiFi card so that gives me an available M.2 Key A/E slot available for use. Which means I should be able to use something like this:

https://www.amazon.com/NGFF-Express-Riser-Speed-Cable/dp/B07KSZ62B8/ and plug in a PCIe x1 USB 3.x controller, correct?

It's funny that this product has an on-board USB port which allows you to keep the USB lines on the slot available for you to use with something else, normally connected to the BT radio on a WiFi/BT card. But it looks like if I remove the WiFi card and use the above product with a PCIe x1 USB 3.x card, it should be neatly accessible by itself via IOMMU Group 19... Hrm...

P.S. I did some pass-through testing with the WiFi card (IOMMU Group 19) to guest OS'es and it works fine.

My only problem with this is I want to utilize that M.2 slot for an SSD.

This adapter should accomplish the same thing, but free up that 2280 slot for an M.2 SSD.