What you need is to scale down the tolerances. To remove the wiggle room.
One of the solutions that does not add a bias that I remember is two identical flat gears on the same axis with a spring that tries to rotate them one relative to another. This removes the wiggle room between this composite gear and the next, regular gear. The motor may have wiggle room, but the gears (which carry angle sensors, don't they?) move without wiggling, and react immediately as you reverse the direction. The load is limited though: the beating surface is twice as small, and the friction is higher.
One could put those in series too and get even less range of motion in exchange for less wiggling.
One could also duplicate the contraption on both sides. Then could replace the arm with cables (under tension) and control motion further down the arm.
Furthermore it seems you could remove the motors from the moving parts?
You could also control the tension of the wires and have accuracy on demand without the wear. I have this mental image of a darts player moving his arm back and forwards to smooth out the tension.
I sometimes wonder if it makes more sense to just use those yellow gearboxes, everyone seem to start with SG90s only to reimplement most of the servo part.
have to share the noob thought because it is funny: You could attach an unbalanced wheel to a motor and induce a vibration to maximize wiggling and frequency across the available w-room.
