I didn't quite understand what to do.
I delete the VEGA driver with the DDU, afterwards I leave 1 or more cards connected?
which bios exactly do you mean, the final version or the beta?
I got the board yesterday and can't get more than one to work. Have also tried the latest BIOS and Beta BIOS with 4G decoding etc. but nothing works.
I also made your IOMMU settings, even without success.
Please ask for help:)
Use DDU to unistall the Vega drivers. Leave them installed in the motherboard. Then try reinstalling.
I actually prefer the next to last BIOS. There are issues with the latest X350 Gaming Carbon Pro/X350 Gaming Pro bios.
Start from the state where you windows can boot up with most cards. Try to add one by one. Leave one card and one card only on the primary pci-e slot on the mb (generally this will make sure windows can boot). check the pci-e riser. Keep trying. As I said, Windows sometimes is stubborn. It took me hours to make it work finally.