If you have an EFI Fiery controller for your DocuColor, driver supported ended years ago and you are correct there will not be a Mojave compatible driver for that printer. For Fiery printers that no longer get driver updates EFI publishes the Fiery Express Driver. That's a bare bones PS driver giving you limited functionality. If you need more than the most basic functions then you have to setup the Express Driver to go to the hold queue and set the rest of your job properties in Command Workstation.
Note: If you update to Catalina you will not be able to use Command Workstation with your DocuColor anymore. It requires Command Workstation 5 which was only published as a 32bit app and those won't run on Catalina.
I am having issues printing on my Xerox DocuColor at work. I was also facing a similiar issue with my WorkCentre 5330 but after downloading a new driver compatbile for Mojave, the issue was resolved. There isn't a driver that I can downloaded supported by Mojave nor any alernatives.
Are there any sugesstions or has anyone had a similiar issue? If so, how did you overcome it?