Glad we can help and your comments/questions help us to better understand our customers needs and importantly, which sails they use on our boards/conditions and any missing products - so it's a 2 way communication or more, as other users offer their experiences too. Very valuable for us!
Quite correct on all the above - the planing threshold can easily be overcome with a more active pumping/sailing style. I often watch people "pushing water" from one side of Lake Garda to another, hooked in, but just not cresting the threshold, which is a shame as with a bit of technique, it's so much easier and less exhausting - which is why I always recommend people to rather invest in some lessons, no matter their sailing level, to improve further. I am windsurfing since 38years and still ask our pros for tips and advice. Not the mention the often terrible combination of gear, making me wonder a bit...
as you mention the different boards could all theoretically work in similar winds. But when you compare within in the board line, with your FreeWave and Grip examples, then obviously one of the boards will have the advantage at least in either the bigger or smaller sail sizes. So the FreeWave 85 will not plane as early as the 95 with a powerful 6.0, but with an active sailing style, you can certainly get close, once you are moving, then the differences become a lot less obviously. The Grip example, the 5.3 is not such a good one as the 5.3 is too big for the 75, it just about works on the 82 and is perfect on 86/92/102. I would say for example that a 5.0 works on both boards, especially on the 82, it's on the limit for the 75. At the same time, a 3.0-4.2 sails work better on the 75 than on the 82. So each board has it's own "perfect" sail sizes, as you mention?
Coming back to you with 70kg - for sure the Grip 75 and FW 85 would be the choices I would recommend for you, 100%. The 82 and 95 would be more for a rider of 78-85kg´s? Hope that helps!