Shouldn't the restricted airspace have a form of an inverted truncated cone (roughly) with increasing altitude limit depending on the distance from the airport?
Another option, for more into the future, is exploiting the aircraft transponders. The more modern ones squawk back the aircraft's GPS position and altitude. They can be fairly easily received by e.g. RTL-SDR dongles. It should not be a major issue to make a less power-hungry receiver and integrate it with existing autopilot solutions, so the drone can keep track of nearby aircraft, predict their direction (Kalman filtering from last n points?) and possible changes (an aircraft won't turn instantly, so it can go only so far off its current direction in the most extreme case), and steer way clear of those areas. Like if the aircraft has a large cone that "sweeps" the airspace clear of drones by telling them to go away. A drone knows its top speed and knows the other aircrafts' actual speed, so knows when it has to start getting away to comfortably make it on time. Together with airspace "blacklists" of the airport corridors, this could provide a pretty safe solution. (The airplanes' forward-facing radars can also serve as additional warning, as an emergency backup; if the drone hears the radar chirp louder than a limit, start quick descent, like a moth hearing the ultrasonic chirp of a bat.)