You are under boarded & under sailed for 13-15 knts which appears to be your predominate wind range.
Cheapest fix is get an 8.0 that rigs on your 460 mast. Next up would be a 490 mast with an 8.5 or 9.0. Third option would be a new board & final option would be both larger board & sail.
Futura or Isonic? That really is a personal choice that only you can answer. Isonic will be faster upwind, faster down wind & have greater VMG. Futura will be smoother in chop, more comfortable ride overall and easier on transitions, especially tacking as the Isonic has very little volume in front of the mast.
I have both. I use my Isonic for racing & long distance blasting. I use my Futura for short reaches with general freeride-different types of jibes &tacks, upwind & downwind 360’s.