Sounds completely normal to me. With a sterndrive, the less thrust you have, the less steering you have. Watch other strendrive boats as they negotiate no-wake zones. They wobble back and forth. But those with "true" inboards with real rudders don't. They can maintain a constant heading. The trick is to compensate half as much on every "wobble". That is, don't over-steer the boat. Unless I'm mistaken, your experience is quite normal.