It is important that you actually perform the motor configuration Auto, not just insert the manufactures values for the inverted status. (for poles you should insert the correct values after auto)
Try auto tuning the PID one axis at time starting from Pitch, holding the gimbal at hand and using 1/4 up from better stability setting.
Also insert the upper support for the camera flash shoe it will make it much better, though probably is not the reason for Yaw problems.
How does the yaw work if you actually hold the gimbal?
Do you have latest FW, some of the older FW had problems with estimate angles from motors.