RE: Drake crossover 26
With my weight of 65 kg, I'm not sure I'm qualified enough with regards to your 81 kg weight, but anyway here is my experience regarding fins for K86 and my NP NR 5.4 wave sail :
1) inboard and front straps setting : the Drake Natural 24 cm wave fin is fine with that sail for waveriding and freeriding. It starts to get a bit too small with a NR 6.2, however.
2) outboard and back straps setting : the Drake Natural 24 cm wave fin feels a bit small when really pushing hard on it (spin-out) when freeriding. Much better results were recently obtained with an old design slalom 26 cm fin that I borrowed from a friend. Not only that fin is deeper, but it has significantly more surface area, providing a much better locked-in feeling on the tail block of the board.
My guess is that for your weight, you'll need a bigger fin than the Drake Natural 24 cm wave fin for freeride blasting and B&J, as you did experience with your previous 25 FS Drake 2002 fin. The new Drake Crossover 26 might be the right call as a do-it-all fin.
As for freestyle fins, I really don't know with K86 as I didn't test any so far. Remember that freestyle fins are low-aspect, i.e., short but have a lot of surface area, so I'm not sure whether a Select FSW 27 cm fin could be spot on or feel a bit too large with your 5.4 sail. I did get very good results with a 25 FS Drake 2001 fin that I used on my previous Carve 111 with my 5.4 sail, but for real blasting, a slalom 26 cm fin was much better again.
Hope this can helps you somewhat...?