Macken, Mojave is an absolute PITA when it comes to security (I’ve dealt with S*** from it myself).
If the message in the Privacy and Security panel isn’t showing up AND the driver for your interface isn’t showing up in the Accessibility section under the Privacy tab, you will have to manually add it yourself. The problem is I can’t tell you where to search for it specifically because I don’t use a Steinberg interface, but had a similar problem with my UA Apollo under Mojave which was solved when I manually added the UA Mixer Engine in the Accessibility section, which is what the OS should have done automatically had it given me a message to allow it to do it, but it didn’t, and it seems that that is also the case with your system.
This is also probably the cause because as you mentioned, reinstalling the previous driver does not work anymore, meaning Mojave is blocking it and you most likely have to manually add it yourself.
Furthermore, Universal Audio is also an identified developer, yet as I mentioned, I also had a hard time with trying to make Mojave accept it automatically.
I don’t think you fully understand how bad the implementation of security features has been done with Mojave. There is no “identified developer” anymore, the title is just for esthetics now. There’s either appstore or good luck trying to figure s*** out. I literally had to allow Logic Pro X in the Security panel. Logic, which is signed by Apple itself and downloaded from the App Store. The security system in Mojave is retarded. And to clarify, your 30 year experience with Cubase has nothing to do here, this is purely a macOS Mojave issue and one that is barely one year old in the audio world.
I get that you’re frustrated, and you have every right to be, but try to calm down and solve the issue logically. The fact that others have updated successfully means that the issue is most probably directly related to your system, and the BIGGEST culprit in Mojave is the Security barrier. Hope this helps.