look at the thread "Big Sur crash restart loop after update" to get some ideas. Remember it is BETA.....

are you facing same problem with beta 6 or only with beta 5?

I had a similar case. I installed Big Sur, I went through the problem of continuous reboot (see other thread), I decided to recover form a back-up I had of Catalina before Big Sur. I ended up with a disk1s1 (empty), Macintosh HD data , another Macintosh HD data (not erasable with Home directory)and the Macintosh data with the OS. What I did is to unmount the two volumes not in use. For the moment I will wait to install next release of BigSur . If you have a back-up prior to the installation of BigSur, the best solution is to erase the disk and recover from the back-up