I think I agree with Joe.
First issue is your sail size. Add at least 1 m2, so 8,0.
Afterwards you still can think about another board.
On the other hand, don't be afraid of an Isonic in this wind range, because there will be almost no chop.
It's only the chop that makes an Isonic harder to ride, compared to a futura.
Isonics get a little bit rougher starting from 20+ knots because of the increasing chop.
Just my thoughts.
IS87 / IS101 / IS122 / US147