Can you run your Mac OS Catalina with Gatekeeper to "AnyWhere" and SIP ( System Integrity Protection) switched off for the time being?
I an running El Capitan, and am able to run elderly drivers for my Alesis io26 audio interface exactly this way.
This NOS (new old stock) interface was declared EOL (End-Of-Line) years ago.
Well, bollocks: it is running fine right now, with Mac OS running rootless!
This is clearly a certificate issue, which NCH software should be able to fix. But woe on Apple for this sandboxing predicament, though!
Please hang on!
I will be running a trial version of Debut Mac OS on an El Capitan iMac shortly and see if it runs. BTW: quite confident it will...