I have a pile of HL tanks equipped with DBC's and RX18's never had an issue as you have described that couldn't be dialed out with radio trim. Are you running the DBC with momentum on? With the momentum on there is going to be obvious delays to stick movements, and it does make the tank harder to control. Try setting up the tank without momentum or as a light tank and see if the control becomes easier.

With 3:1 gear boxes an RX 18 with a DBC with momentum should be smooth and true. I have lots of video showing this set up in motion, in front of a camera in a tight space and have no problem with control.