Wide slalom boards = outboard straps. Get a bigger fin and you will need more pressure on the front foot to keep the board from flipping. Foot straps too big? Even with my somewhat tight straps, I have to lift my toes almost continuously to keep my feet snug. I use a 42 cm Techtonics fin with my 7.4 race sail on my Starboard isonic 111. I weigh 170 lbs. The fin also works fine with my 8.4 and 6.6. A little big with the 6.6.
Apologies for not reading all replies... happens regardless of wind angles? Have you tried different lines positions? Too much pressure on back foot = move them back.
You have several problems by the sounds of things.
1: move footstraps outboard.
2: Keep mast track at 135cm from back of board. SThios should equate to centre of track.
3: Don't "canter" the rig to windward too much when overpowered as it will lift you too much and pull front foot up. Just sheet out more and keep rig more upright.
4: harness lines should be 26" long.
Your gear is way too big for your weight. At 74kgs my biggest sail is 7.0m and 97L isonic. Planes in 12knots easily.