Tough call. For me at 72kg, the Quad 76 in fact works as my biggest board, I tried the 81 too but it offers no extra for me. This is with sails up to 5.6, I suspect the 81 take a bit more sail.
Based on that and that you are not that much heavier, I believe the 81 is gonna work quite well as a light wind wave board, and you will have just the perfect overlap so that in marginal conditions around 5.0 you can choose between the 71 and 81 depending on how much "board power" the conditions call for.
But I also think the 6.2 might work better on the 86. Both from my own experience and from talking to others I have the impression that if you overload the quads with sail, they will require a more careful technique to plane early. Once going they are still effective, but there can be a bit of a draggy feel you have to push through before. So if the best possible performance with a 6.2 is important, the 86 might be it. I haven't used it so I can not say exactly how it works. One thing to consider though is that I feel the Quads all are very "sail economic" in the sense that even when a bit underpowered you can still go upwind extremely well which means you can get around the break and catch the good waves with a smaller sail than normal if you have to. So maybe you will not need the 6.2 as much as you used to?
So, no straight advice for one or the other but some thought to consider.