I think we have the same issue.
Opencore boots fine, I managed to install until 12/13 minutes remaining, after that tries to boot, but crashes.
Then manual rebooting opencore loads fine there's a option for bigsur macos install select this and at the phase that the apple logo is already up the screen it crashes and can't even see the errors, because it's too quickly something with "sigakill: shutdown...!!!.
Been this week trying to figure this thing out, but no show!
Any clues?
EDIT: Since I can't make any usb to boot in my system, i booted from empty internal ssd, only with EFI populated, after that i use the usb to make the installation to the internal SSD. Could this be the issue?
HP XW6600, 2xXeonE5430, 16 GB, 256GB SSD, Nvidia 710GT.
EDIT2: I was able to get a picture of the final verbose, just before the screen become black. Can anyone make something of this, it's not very clear.
Screenshot 2020-09-11 at 17.33.35.zip