I use a iFligt iPower GBM4108-120T 24N22P for yaw axis together with a Sony A6300 and a 16mm lense. The torque of the motor seems to be ok. I power the motor between 70 and 140 and the momentum is completely ok. The I value is best between 0.02 and 0.05, so that is not the problem. But I donn't get the yaw axis stable independently of choosing any combination of P and D values. On the left and right edges of the video I always have more or less shaking effect of the video. I tried more than 100 combinations of power, P and D values without satisfying results. The P values I tried are from 10 to 100, the D values from about 30 to 180 and the power values from 70 to 140. A power value more than 140 causes vibrations. So is thre a way to find the optimal settings or do you think that there is another problem?
Is gimbal producing high frequency vibrations? If yes, maybe you can try using Notch Filters. By using Notch filters I managed to use much higher gains that resulted in noticeable better stabilization.