OK your weight is 85 kg. It is def. not tail walking with your weight and starting at 15 knots.
It’s a skill/technique question in my opinion.
I own a Futura 144 and have sailed a lot in everything from 10 to +30 knots flat-water, heavy chop side shore and onshore conditions. This fin is super fast, no spinout, best stock fin I have owned.
I guess you don’t get your weight off the board, standing too hard on the windward rail, or sheeting out to much. You have to sail the board flat. Get it planning a bit downwind, harness, straps, and sheet in by using your body weight, and tilt the rig back when gaining more speed. Lock the board with your feet in the straps, weight on your toes, not your heels. Then the board will free, fly on the fin and you go fast and stable; just trust the board and the fin. I think you are backing off, let it go even if it feels scary.
Sure 6,5 – 9 m2 sails are the best sizes, but I have sailed it with 5,4 and 5,0 wave sails on the stock fin and it worked OK.
Don’t think you need a smaller fin before you are close to 25 knots.
You own the best freeride/freerace board and stock fin you can buy.
Next fin size down 46 or 44 cm slalom or free slalom type, brand ? depends on how much money you want to spend.