Help! How to clean up apparent partial Monterey install and revert to Big Sur? Bits of Monterey running?

If you still want to upgrade to Monterey please read the following:

I've had better luck from the Safe Mode because I evidently have some 3rd party software that interferes with the installation so try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 3-5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the System/Software Update preference pane and begin the download.

#3 - run the installer from the Safe Mode.

The performance issues your iMac was experiencing is most likely caused by the Symantec software. Anti-virus apps, cleaning apps, and third party security software are not needed on a Mac as they interfere with the normal operation of macOS and usually causes more problems than they solve. macOS already has great built-in security especially when combined with users practicing safe computing habits as describe in this article written by a respected forum contributor:

Effective defenses against malware and other threats - Apple Community

It is not clear if you were already running Monterey and attempted to install an update patch, or whether you were running Big Sur and tried to upgrade to Monterey. I believe you mean the latter.

See if you can boot into Safe Mode which will disable third party software from automatically launching during boot and login. If so, I recommend you uninstall the Symantec software by following the developer's instructions. Depending on which version of the Symantec software you are running, you will either need to get the instructions from Norton Life Lock or Broadcom (Symantec recently split their company and sold the enterprise part to Broadcom which makes the Google search results look worrisome).

You can also try installing Monterey again while booted into Safe Mode to let the install process complete (you may want/need to boot into Safe Mode again after the installer reboots the Mac in order to disable the third party software in case it is still causing a problem).

AFAIK, the Monterey install process should have only created another APFS snapshot for Monterey and should not have affected the Big Sur install, but I am not certain.