I've done a bit of bulk parts processing in my career. You don't really need the paddles in a rotary tumbler. The cleaning action is performed by the parts sliding down the mass of parts/media. You want to make this face as long as possible, so you want your vessel to be about half full, so the face is equivalent to the diameter of the vessel. The paddles effectively shorten this distance.
Rather than paddles, use bumps, so the parts get carried up the side, of the vessel, but don't get dropped.