> I have scoured the web for any way to use a Nikon as a webcam on macOS (over usb, so without dropping $120 on a camlink which is sold out everywhere now anyway)

check out https://news.ycombinator.com/item?id=23409540

I misplaced the usb cable that came with my nikon d5000 but I was able to find a replacement on amazon yesterday that should arrive later today.