I've got a Lessacher Carbon Duo Weed 28s, which is a bit more svelte in outline, as it offers somewhat less area overall. Although I have been using it on a Mike's Lab, the board is of a similar volume, and I'm the same weight as you. I use this fin up to a 7.1 with complete confidence, without any spinout or crabbing, and it jibes so well. It could be that the larger standard Duo Weed 28 would work quite well with a 7.8, as Roger has recommended.
I also have a 24cm Chamaleon that I've used with the 7.1, and while it had excellent drive, free of spinout and crabbing on reaches, it didn't like to be driven hard into jibes and would tend to break loose if suddenly pushed too much. But to be fair, it's a very small fin for such a large sail, and I'm sure that a larger size would most likely solve the problem.
No doubt, whether you go with the Duo Weed or Chamaleon, I think that Lessacher is offering premier weed fins like no one else out there. You're clearly on the right track.