Try the mast track in the middle, or around 132-134cm from the tail. I think a slightly bigger fin might help too. The original fin is very small for its size, so to say. Moving particularly the back strap a bit forward might help too.
After that is mostly about technique. M advice would be to give the board a fraction longer time to settle in the top turn. Ie don't hit it with full power right away, but ange it up in the turn and then start to drive it. This will secure some tail grip before and settle the rail which will then hold on much better.
To do this, sail handling is essential and if you take some extra care to open up the sail before setting the cutback rail fully, you will probably get a cleaner carve.