I have the same issue. One of my MBP's just doesn't want to update. It gets stuck every time on "20min remaining" while "preparing macos Monterey 12 beta 2". I've tried a million times already for the last week. No idea what is causing this. What I did notice though, when I try to boot the laptop into "safe mode", it actually doesn't want to do this. It ends with a black screen, a loading circle, shows a split second a cursor arrow in top left corner, black screen with only loading circle in the middle again and this loops and loops and loops. So trying installing this in safe mode is no option. I already did a nvram reset, ran first-aid on the disk and all seems ok. There is a ton of space free on the disk.... no ideas anymore.