So, after trying a bunch of things, I was able to recover by doing the following: boot recovery (Command-R)and completely erase the drive disconnect any time machine backups restart and do internet recovery (Option-Command-R) install the default OS for your hardware (I ended up with El Capitan, sheesh) work through installing OS updates (for me, Catalina, and then Big Sur) until you are back to Big Sur Beta (it installed 4, luckily) assuming you have a backup (which of course you do, right?), use migration assistant to restore your account(s) Turn off auto update and DON'T update to Beta 5.