Ok so it’s a little bit experimental and not fully tested so can’t verify if everything works
But, after flashing the image to your USB, open the USB drive in Windows Explorer/Finder and replace the contents of cmdline.txt with:
console=serial0,115200 console=tty1 root=/dev/sda2 rootfstype=squashfs,ext4 rootwait
When I do that to point to the USB rather than SD it seems to boot and you can see PiFi 5 Setup in WiFi Settings
Again, this approach is not fully tested. But it does seem to boot so is worth giving a try