If you have a proper steering vane, you won't have to worry about the strength of your tiller pilot in heavy going. Those are the conditions when the steering vane typically will work its best, and the tiller pilot won't be used.
A lot of folks have hooked up tiller pilots to steering vanes,in order to provide light air and compass course-keeping capability, with low power consumption. But I seem to recall it's most commonly done with servo-pendulum types. The tiller pilot replaces the input from the windvane, and the servo pendulum provides the power to turn the main rudder. The servo pendulum is usually kick-up style.
Personally, I would not want a permanent trim tab affixed to the main rudder if I had other options. I would want the ability to get all the steering vane components out of the water when I didn't need it.
Hartley sells plans for a DIY steering vane:
Hartley windvane self steering plans