Same issue with 2014 iMac 27”- very frustrating!

I know this is old, but I just had the same issue with an old 2014 Mac that I was updating this week. (I wanted to update due to recent security fix, even if it means losing old Microsoft office.) After installing the update, my computer would not boot and was stuck at 25/30% on black screen. I waited hours and then tried to reinstall BigSur overnight with no luck. Then, I decided to try the disc utility (and back everything up before I did since before the update I didn’t think it would have trouble!!). It did need some minor repairs. I then reinstalled Big Sur, hoping that would work, but it was still stuck. I also did opt + cmd + p + r, but still nothing. Finally, I restarted it into safe mode (shift on start), which made the progress bar go farther, and now finally it is unstuck. Hallelujah! Though now it is making me recreate my account totally and was going through setup part very slowly too. My computer was working fine before- it was slow but at least worked well enough for browsing. I’m just glad I didn’t lose my machine or files (at least as of now) in this process. It is finally back to the normal desktop screen, but I haven’t restarted it yet.