After I installed the recent update on my iMac Pro running Mojave, the entire system went bonkers. Slow startup, freezing screens, won't launch apps...just a mess. I read this was a result of the faulty update. How do I remove that update?

You do not need to open the App Store to update, in Mojave updates are done via System Preferences> Software Update.

Try booting your mac up into Safe Mode and then try the update.

How to use safe mode on your Mac – Apple Support

Safe Mode takes a long time to start up so be patient.

Did this happen after the 2020-005 SecUpd for Mojave?

Try reinstalling this in Safe Mode, but not using Firefox...

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Safe Boot, (holding Shift key down at startup).

No.

Safari 14 was released before the Security Update 2020-005, after that Security Update was released people experienced problems with their macs. Apple released the Supplemental Update to iron out these problems which also included fixes

for Safari 14. After installing this update there seems to be less posts about malfunctioning macs.

If you can get yours booted to Safe Mode and install the Supplemental Update hopefully things will improve for you.

You can reinstall Mojave or use a backup to put back Safari 12. I have done both.

Have you installed the Supplemental Update that was released a few days after the Security Update, this is supposed to

sort things out.

Go to System Preferences> Software Update.

This update installs Safari 14 which does not still work well with certain web sites. I kept my previous version 12+ and all works fine.

Great to hear, take care my friend! :)