Sorry friend, going to shoot you down.
build a quadcopter with rotors that can tilt to increase the drone's flexibility.
The Stingray 500 is an amazing piece of tech. It is a rotorcraft (heli). So this is possible for tilt of this nature. Caveats include complex engineering considerations, considerable drag, and the build wants a single power plant like ...
You don't say what firmware you're running on the KK2.1, but it's probably open source and available for you to edit. Have you seen OpenAeroVTOL ?
Most modern flight controllers run BetaFlight, which is also open source, so most of the hard work has been done for you. You'd just have to modify it to add tilting rotors.
You probably won't find good options ...
It's possible your ESC is not calibrated to the PI's output.
Most ESCs are calibrated by this procedure (using a controller, you can translate this to your code):
Move throttle stick to Max position.
Power on model. You will here the battery cell count beeps, and a periodic single beep.
Move throttle to Min position. You will hear a beep confirmation.