Yes, a smaller fin will help when it gets windy and you are going faster.
32-36 cm sounds about right to me.
52 Kg. (115 lb.) is pretty light for a 40 cm fin on nearly any board.
You don't have the weight to control the fin.
When the wind comes up a smaller board with an even smaller fin (down to 26-28 cm for 4.0-4.5 m2
Also yes, a more curved outline will help with your carving jibes.