RE: Isonic Choice
I ride an iSonic 122 with a 7.6 and 6.6 MauiSails TR2. I am 180cm and 70 kg. I have sailed the 7.6 with the smaller supplied fin in 18-20 knots, and the 6.6 in more than 20. For me the board is quite big and is really good in the lighter wind range, under 20 knots. But for your weight, IMHO the board is just about perfectly sized to overlap with your FW board.
With your extra 16 kg over me, depending on your skill, water state, and sail, I would think you could use the 122 up to 25 knots EASILY with a 6.5 to 7.0 race sail.
You should also be roaring around in 12 knots with around an 8.5 or 9.0 sail. If you have never ridden an iSonic, you are in for a treat.
My 2 cents worth. Hope it helps.