I have been using Connect My Tablet to keep my Cintiq going, but this fix is far better since you can now make changes in the Wacom Preference Pane and not have it immediately crash.
If you have been using Connect My Tablet, you need to go into Open System Preferences -> Security and Privacy, and turn off AEServer, as shown below, otherwise it interferes with the patched Wacom drivers.
You can either manually replace two Wacom drivers if your Driver is already installed, or use the custom “Install-Wacom-Tablet-6.3.15-3-patched” installer if you remove the original drivers first (back up your settings so you can re-import them).
I just used the patched installer on a fresh install of Mojave that had no tablet drivers installed, and it worked perfectly, and I used the Wacom Tablet Utility to load my preferences I backed up from my main hard drive (Catalina), and my Cintiq is working perfectly.
I gave away my old Intuos 3, so I can’t test it with this, but it should also work with this and other models.
Fixed Cintiq 21UX Driver