RE: why not ask the pros?
With your 65 kg, I think you're hitting the max usable range of your combo Go139 +6.6 sail up to 21 knots. You might be better in control with a smaller fin (something about 40 cm or less). For up to 25 knots, this is gonna be pretty challenging with your light weight and I think you should be on a much smaller sail, e.g., 5.5 sqm with an even smaller fin than 40 cm.
FYI with my 65 kg on HS105 : race 6.6 sail or wave 6.2 sail with 32 or 26 cm fin in 15-20 knots of wind. For 20-25 knots of wind, I use a wave 5.4 sail and 26 cm fin on such a large board, which is becoming the upper limit before damaging either the board or myself.
To further prevent catapult, it's super important to downhaul your sail to the maximum to be able to release extra power in sudden gusts.