iMac failing to complete upgrade to Big Sur during restart; endless restart loop

I've been trying to upgrade my 2017 iMac from Catalina 10.15.7 to the latest Big Sur update. Twice have I tried installing it and twice has it failed, but not at the usual stage that most other people seem to have experienced issues at.

Everything is fine up until my iMac reaches the restart phase. The Apple logo appears and the progress bar underneath will load a small bit, then after only two or three minutes the computer will restart again. No further progress is ever made and this repeats endlessly.

Of course, this was pretty alarming when I first tried to upgrade because no number of restarts would start up the disk. Curiously, holding Command + R does absolutely nothing. I have had to enter Recovery Mode before and yet it just goes straight to the progress bar as though I'm not even holding the keys down.

Luckily, holding Option to prompt the Startup Manager works, and then selecting the hard drive starts it just fine, Big Sur is not present whatsoever as my iMac is still running Catalina. If I try to restart my computer the progress bar issue persists, but reinstalling Catalina fixes this issue. Unlike other troubleshooting threads where other folks just run out of memory, there is no error presented to me regarding the failure to update.

TL;DR I have absolutely no idea how to safely upgrade to Big Sur, and I don't know if the restart loop would suggest a hardware issue. Insight would be much appreciated.