Its OK Zoltan , we know each other .

The fuse or safety valve is only when you hit ground.
All the other forces , no matter how extreme or heavy you are should have no pressure on those bolts . None ! OK maybe a smidge in drag through water only. If used in a Tuttle box.
In a Tuttle fin to box connection the tight parallel sides take all the sideways pressure , not the bolts . In a fin or foil this is where the forces are.
The angle on the ends of the fin / box keep the fin / foil parallel to the board. So only a small amount of tension is needed on the bolts .
What your doing is relying on every force , sideways , back , twisting and imaginary , hanging on to two weeny 6mm threads .
So , I believe , if the foil is mounted in a Tuttle box there should be no problem with your weight or radicalness and the 6mm bolts will be fine.