iMac endless restart loop, unable to enter Recovery and Safe Modes

Two days ago I asked a question about my iMac failing to upgrade to Big Sur while restarting, but now I'm trying to diagnose whether or not my iMac is experiencing a more concerning hardware or software issue.

TL;DR my iMac will restart endlessly after a power outage or manual restart, but fortunately I can get it to boot up normally if I use the Startup Manager. During restarts, whenever I attempt to enter...

Recovery Mode: does absolutely nothing - Apple logo appears as if I'm not holding the keys down

Safe Mode: abnormal behavior - causes an instant restart as soon as the Apple logo appears

Diagnostic Mode: reports no errors then offers restart or shutdown as usual

Startup Manager (Option): selecting the macOS installer just resumes the restart loop, selecting my startup disk boots it normally

I am trying to install macOS Big Sur because I have been experiencing very frequent crashes with my software, and working with its developers has led nowhere but presuming a hardware issue on my end.

Troubleshooting other help forums suggest a kernel issue, but I don't know enough about hardware to verify this, and the restart issues don't really point to a kernel panic. I have already reinstalled macOS Catalina (which is possible oddly enough, and not Big Sur) but all these problems persist.

Help would be very much appreciated.