Hi, have you googled/researched what other model rocket people are doing?I think most of them that do flight control use small tabs on the aft fins, or small forward tabs with small servos. This gives guidance that also works after the boost phase.
Can you mount the engine so that it is gimbaled in a way that there is no force needed to tilt it??
With this size rocket, I assume it's solid propellant?
This is sounding pretty mechanically complex, compared to forward fins.. Where would you mount the servos? Forward or aft of the fuel tank? How would you link them to the engine?What is the run time of the engine? It'll be HOT back there.What's the current possibilities for a Monopropellant motor? Ethylene Oxide? I hope not NitroMethane. (My high school got a visit from Guys In Suits after a newspaper article about my friend and I building a CH3NO2 based engine).
Have you actually purchased the hydrogen peroxide for your project? Your best bet for controlling the rocket is with a standard RC setup. Model rocket forums probably can provide you quick answers.
To guide the rocket, my plan is basically to use a gyroscope to sense when the rocket is not going straight up, and to rotate the rocket motor accordingly