A relatively easy fix to make a distinguishable looking techpriest from the Dominus kit is to flip his torso around. It removes the hunchback posture and makes it look like a different model.
Chop the body segments in half at the waist, where the armored torso section begins. Sand them flat, flip around, and glue into place. The arms will have to be altered as well to match with the new posture, and you’ll need either a new head or to chop up one of the old ones. With a bunch of Mechanicus kits I’m sure you have some spares.