Hi Kokomorbt.... and greetings from Ireland... 🙂
I have had an awful experience also with that whole Security Update 2018-002... actually, I'm now stuck in a ridiculous situation that I can't even use Recovery Mode to fix the issue as my MacPro 12 Core (Mid 2010) is now incapable of automatically restarting or shutting itself down so I can't even get it to Re-Install High Sierra or upgrade the Firmware to go all the way to macOS Mojave.
I have never used Terminal to fix an issue... but I'm tempted and terrified at the same time to try out your solution... is it just a matter of copy and pasting your steps into the Terminal and that's it... problem solved ?
I have been speaking with multiple Apple Tech Support people and no one has a clue how to resolve this and a Terminal-based solution was never ever ever even been mentioned.
Did it 100% work for you... and what did you do... go back to High Sierra as you were before the update appeared or go all out macOS Mojave... I have a new Sapphire PULSE RX580 Graphics Card good to go and all my workflow apps are 100% macOS Mojave compatible... so I can go Mojave if that helps get me back on track and up to date again, and 100% up to date with my Apple OS software.
Please advise and revert when you get an opportunity.