I wonder if you could dangle the star tracker below the drone on a long string, decoupling its attitude from the attitude of the drone. A kevlar or spectra string capable of supporting 100 grams would be 20μm in diameter; 3 meters of it would weigh a milligram, which is significantly less than gimbals. A small weight a couple of meters below the star tracker on a carbon-fiber-composite rod would seem to be able to stabilize its attitude further except in yaw.