Steering servo to replace the RTR.
Stronger front and rear shock towers.
Front and rear braces won't hurt but they'll put more emphasis on suspention setup.
Front and rear sway bars would help flatten the chassis in high speed turns but also limit the suspension on very choppy, rough ground.
The diffs are fine but you need to check them for play and shim the diff accordingly to mesh the ring gear with the pinion gear.
You already discovered the spur gear. Use a piece of notebook paper when you set the mesh between the CB and spur, you do want to have a tick of free movement and be sure it turns all the way without any tight spots.