Can’t boot after upgrade 12.5.1 unsuccessfully

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Does the issue present in this mode ?

3 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

4 - Trying to recover from a Time Machine Backup will only reset the Personal Data. Starting in macOS 10.15 Catalina - TM Backup does not contain a Full Version of the Operating System

At this point and after attempting Safe Mode and still not working but sorry it appears time for a List with the computer to >>

Find an Apple Authorized Service Provider and / or Visit a Genius at an Apple Store where they ave the Special Hardware Tools and Expertise to evaluate this computer and may be able to fix this problem

For booting an iMac which has a Bluetooth Keyboard - it would be better to use a USB connected Keyboard for this Safe Boot and the Keyboard Directly connected to the Computer

Reason, the Bluetooth Kernel in the OS may not load in time to allow the SHIFT key to function. Thusly, can not boot into Safe Mode.

I always keep a cheap USB Keyboard on hand for just these Special Occasions