I'm not a slalom sailor and I have little experience with this, but my guess is that the most effective way to trim away that excessive back foot feel that make the front foot "loose" in the strap might be by some sail tuning, most likely more downhaul. That will open up the head more and push sail power lower down in the sail. And since you are sailing the sail raked back, the effect will be that the power moves a bit forward. This might give you a more balanced stance.
Boom and mast track can change your stance too, but since you tried that, the downhaul trick might be worth a try.