Not on team and apologies if my answer is too simplistic or insulting. But as an experiment just try moving your harness lines back approx 2cm on boom.(At point where you think they are a bit just too far back.) Try and go on speed run with rig slightly forward; position you`d put it in to just start bearing away. Note speed now.(If you spin out) Even with a poor fin it should be generating enough lift to not spin out at 30knots. Its been my experience that most spin out problems are rig and or rig position related. As long as you aren`t overloading fin and its not aeration (overloading fin at 30k is difficult !!)
Good luck !!!