Hey Dave! Apple is getting new OS update and some software is forcing me to update above Catalina. I’ve made some research and found some possibilities:
First is Ploytec’s most recent project that you mentioned, the Ploytec revival, a USB highspeed converter for Xone:4d 2d DB4 & DB2 (without midi support yet)

Now, for the software possible solution, has anyone tried OpenCoreLegacy to install a patched macOS? This is intended for older MBP to run newer OS, but as far as I know this also enables legacy drivers to work seamlessly on newer OS by patching kernel and security issues

Could this work? Has anyone tried this?