So, I've had the same issue on my iMac 2017. I tried all suggestions, I got the furthest with @Charaiveti's suggestion until my mouse stopped being accepted as an accessory. So I couldn't choose "Reinstall Big Sur". I connected a wired mouse and had about 2.5 hours of "support" from Apple, essentially having me do the same things over and over. I'll add that about an hour of that support was talking me into the idea that my mouse was dead and I needed a new one. We reset SMC and NVRAM. When I was finally able to choose "Reinstall Big Sur" from the recovery window, it tells me "This opperation couldn't be completed. Permission denied." Quick note, I am the admin user. So I reinstalled Catalina (and by the way my mouse started working as soon as I reinstalled) and tried to install Big Sur again. Same issue. My load bar sits at about 1/5th of the way for hours and hours. I think I'll just wait until another version other than 11.0.1. Unless there are other suggestions.