OK. I think that puts you in a position where both the Q76 and 81 will work. The 86, I think, will be to big. Unless you're standing completely still, I think you will find the Q76 as floaty feeling as the NW81. It's one of those boards that just stays on top of the water despite not having a lot of volume. And both will keep speed on the wave equally well at your weight and with a 5.3 sail the 81 will not really plane earlier (ie 5.3 is well within the sail size spectrum for both).
So in the end it will be about the 81 having a little bit of extra float and "corkier" feel which will give a tad more comfort in the lightest winds. But the 76 with its shorter length will have a more compact and looser feel to it which also makes it easier to use it in higher winds.
I have both, but for me (at 72kg) the Q76 is definitely as big as I need. I've had a lot of light wind sessions, but even then just prefer the 76. But you are a bit heavier and it's also kind of personal. But given that the NW81 (a narrower board) felt pretty big enough, I think you will be happy with either Q76 or Q81.
And I can pretty much guarantee you will not have problems with either keeping speed or sliding out when you don't want to.