RE: Worm Gear Drive Servo
To increase the torque, but with reduced speed, I think that you could try to modify a standard servo in this way:
Drive a threaded rod with the servo axe, remove any stops inside the servo.
Connect your control surface to a bolt driven by the threaded rod. The bolt should be designed to limit backlash.
Do not use the internal servo, but an external one connected to the surface position. I advise to reduce the length of, and carefully shield the wiring.
Alternative: use a DC motor with reductor, and design the electronics.
Philippe