Mac OS 10.15.4 supplimental update issue

When I download and try to install the 10.15.4 supplemental update, of course the system wants to do a restart. When the restart occurs, I get a Zero on screen with a slash through it. That remains on screen for 5 to 10 seconds, then it is replaced by the Apple logo. At that point, nothing else happens. I've let the computer sit for 20 minutes, and there is no progress.

Thank goodness I can use the option key and boot off my Carbon Copy Cloner backup drive and use CCC to re-install the OS on my internal drive.

Software Update in my system preferences still wants to install this update, but after getting the dreaded zero three times and having to use my backup to recover, I'm at a loss.

CleanMyMac X tells me everything is clean, no malware, and I'm good to go, but so far no love.

Ideas?

iMac Retina 4k, 21.5", 2017

3.6GHz quadcore i7, 16GB ram