Monterey OS update issues

I try to avoid updating the OS on my desktop iMacs as long as possible since it always seems to result in unintended problems (need to download or purchase updates to other programs that I run). I have recently been receiving notices about downloading updates to the OS--which I always reply with "remind me later". Two nights ago I believe my 2020 iMac tried to download and install the new Monterey OS and in the morning I had a "brick". Computer was powered down and every time it tried to start it would progress to a point beyond the Apple logos, then "sigh" and shut down. I have tried most of the fixes suggested on line--holding down Option key and trying to boot from my back up boot disk on a flash drive plugged into a USB port. Using this approach I have also modified my security settings to allow booting from a system not on the computer's hard drive. But the program now tells me that the boot disk has been improperly "partitioned" and offers me only to erase the disk and reinstall the OS. I have also tried booting up in Recovery mode (Alt + R) and tried to repair my hard drive with the repair utility--it tells me that the drive appears fine; and downloading the OS Big Sur (I presume over the internet). I live in a rural area and the best internet download speeds I can get are 34 MBS. I have tried this approach about 4x. It initially tells me to expect a download time of 2 hours and 22 minutes, then this keeps getting longer until it just fails. I have been trying this over night and woke up to find it still "working" but predicting 54 hours and 12 minutes remaining. Any suggestions on what I can do to fix this? One web site suggests using a second computer and woking with Apple Configurator, but my second computer is an older iMac operating off Mac OS High Sierra (10.13.6) and Apple configurator requires 10.15.6 or later OS. I am at the end of my rope. This was a pretty expensive computer. Too expensive for a paper weight. Much thanks for any help you can give.