It’s likely the mobo. A Ryzen processor and a nvidia 3070 isn’t so uncommon. And you already said you updated the BIOS and put quite an amount of effort in.

If you can return it or have a friend in need of a mobo… I’d swap it… However, there is still a possibility that someone reads your post on the weekend and happens to know something more. Or you use Linux in a VM from within Windows. That may work, I don’t know you, I wouldn’t do it since I use Linux 99% of the time.

Only thing I would advise against is hoping the issue will someday get fixed. In many cases it never happens.