Big Sur update; startup disk won't load

I am trying to update my MacBook Air with the new big sur update. It won't work. Every time my computer restarts, it will take 30mins to update/install, then once it does, I am notified that there's been an error (no error number or any specific details). I click OK and a "choose startup disk" screen pops up, but no startup disk appear and the "thinking" circle keeps moving like it's trying to find a disk but it never appears. If I quit the startup disk, my macOS update assistant remains on the screen with the progress bar empty and not moving. Attached are pictures of these popups.

I've shut down/restarted a handful of times, each time the same process (which is infuriating because it takes 30+ mins). I've tried going into safe mode but I never get to a login screen.

I did have a USB-C cord initially plugged in to connect to a second monitor - not sure if that caused an issue because I think that port (lightning bolt with arrow) also can connect to other hard drives? Regardless I can't seem to get anywhere and I'm fretting because I need to be able to access my laptop. Please help.