First, I don't know Futura so I can't comment.
However, I own an iSonic 117 Wide (carbon 2012) and it's a sinker for a 105 kg buddy + 9.5 m2 sail. The true volume of 2012/2013/2014 iSonic 117 is around 120 L, so it might be a semi-sinker or barely a floater for your 98 kg + 9.5 m2 sail.
Pairing an iSonic 117 with a 6.7 and a 5.8 m2 sail is a bad match IMHO. A 7.7 m2 sail is a killer combo on iSonic 117 wide in 5 Bft but I could not imagine going in 6 Bft wind + 6.7 m2 sail with such a large board.
Reality check on my home lakes is that most of my 100 kg buddies which are using a 85 cm wide board (iSonic 137, JP 140, Fanatic 135) are also using a 9.3-9.5-9.8 m2 sail and a 48-50 cm fin. Planing as of 11-12 knots of wind with lots of pumping (4 Bft). A 7.8 m2 sail is way too small in 4 Bft wind for a 100 kg rider.
Another 105 kg buddy is using an UltraSonic 147 with 9.0, 9.6 and 11 m2 sails (sustainable planing wind range between 3 and 5 Bft) mostly on lakes.
In summary, width AND volume are key factors for a ±100 kg rider to enjoy windsurfing in 3 or 4 Bft winds on lakes.