iMac update for monterey 12.6 keeps failing

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.

Startup in Safe Mode and try the update.

Start up your Mac in safe mode – Apple Support (UK)

If you have any antivirus, cleanup or VPN apps installed these can cause the problems you’ve encountered and need to be uninstalled. Safe mode runs without background tasks so you may be able to install the update from there.

Same problem, only worse…..gets to preparing update them give internet error (bogus as speedtest confirms 300+mbps), then system won’t restart…..either goes to logo and progress bar that freezes (waited 3 hrs), or gets to home screen with dock and no menu bar….totally unresponsive! Working on third support agent with no solution.