From your description, it sounds like the starter pinion is not engaging the flywheel/drive plate ring gear completely and spinning against the teeth. Your latest failure indicates that enough has worn away that the engine cannot turn to a good position on the ring gear to allow the pinion to even mesh. You are probably looking at a new starter at least. Your mechanic should have a look at the entire ring gear to see if there are badly damaged teeth. If so, you may need to replace the flywheel/torque coverter drive plate.