So as it stands you have Big Sur partially installed, but no way to proceed with the remainder of the installation, yes?
You may need to wipe the drive that currently contains the unfinished Big Sur installation, and start again.
Might be worth looking at re-creating your Big Sur installation USB too, i.e. remove the installer from your Mac or other hack, download a new Big Sur installation application, and after erasing the USB drive installing Big Sur installation application to the USB drive. Just to make sure the Installer is not causing some of your issues.
You may need to wipe the drive that currently contains the unfinished Big Sur installation, and start again.
Might be worth looking at re-creating your Big Sur installation USB too, i.e. remove the installer from your Mac or other hack, download a new Big Sur installation application, and after erasing the USB drive installing Big Sur installation application to the USB drive. Just to make sure the Installer is not causing some of your issues.