RE: Long and Short Board suggestions
To some degree, I agree with Rod_r here, and since I've sailed both the Kona and the Phantom, I may have a slightly unique perspective on these 2 excellent new boards.
There are things the Kona probably does better than the Phantom, but the opposite is also true.
For super light winds, the Kona's longer, narrower shape may have some advantage over the Phantom.
In windspeeds above 10 knots, the Phantom will probably plane up (with the center board retracted and a large fin) at least as early if not earlier than your older Free Formula 198 as it's a very efficient new shape, but the overlap between the FF-198 and the Phantom would make one or the other very redundant.
As far as a larger Carve (to fill the gap between your FF-198 and the Hi-Fly) that could work as well, but again the overlap is going to be problem.
With your gusty, light wind conditions, it's going to be quite difficult to find a mix of boards without some degree of overlap, and replacing your current boards (I know you don't seem to want trade them in) might allow you to find some different board characteristics that will allow you to progress.
Sorry I can't provide a better solution.
Hope this helps,