Thanks guys.
I read the article and your advice and it seems that I need to change quite a few things.

>>>fin will make a HUGE difference !!
This is the fin I mostly use. It's a shortened 36cm JP Weed (down to around 29cm now).
I can see it's not good and should probably get more efficient fin.
It doesn't collect weed, and doesn't slip much, but I'm struggling to get up wind, and as I get close to 25 knts, I lose control unless the water is completely flat.
I should start by getting a speed fin for a 100L board combined to 7m sail. Then apply your advice one at the time.