Do you have a reliable backup?
Make sure you are following the Safe Mode boot procedure exactly. What happens when you try to Safe Boot?
If you have a reliable backup, you can do the following which I did a few years ago when I had the same issue with a Security Update with High Sierra.
[Follow this only if you have a reliable backup] Boot into Recovery (holding COMMAND-R on boot).
You can try (a) Disk Utility to repair and if that does not help then (b) reinstall the OS (without erase or reformat).
If neither (a) nor (b) resolves this, again, only if you have a backup, you can select the option to reformat and erase the drive, then reinstall the OS, which I expect will be Catalina. This wipes the drive, you must have a backup. After the MacOS install is done, on first boot install one new user, call it ADMIN, with administrator privileges. Nothing else. Then apply all updates available through Catalina. This will include the problematic Security Update, which will install because you have a factory fresh MacOS at this point with nothing else. [If it fails again then you need to have your hardware tested.] After all this, reboot, verify all updates have been applied and verify proper functioning, and then use Migration Assistant to migrate your accounts and files from your backup. You will have the option to migrate everything (files, settings, applications) but I recommend bringing over only user files, then reinstall only needed software and apps from scratch, re-establish settings from scratch. This minimizes the chances of re-installing the item that conflicted with this Security Update and caused the freeze.
