Chris and Marko has good points, what to do about your "floating" front foot problem. For slalom sailing it is one of the most annoying things to experience.
The Isonic 86 is a very comfortable and fast board. There is nothing wrong with this board. It has a good balance on the foot pressure (front foot/ back foot)
I think your problem is more (90%) towards your rig/sail. If the balance in the rig is wrong, you will get this problem you are talking about, where you have to much pressure on the back foot and nothing on the front one.
Best thing now is to try another rig/sail from some friends, and you more likely quickly fell the different in stance/ balance that you get on the board, from whatever sail brand you try. They all have slightly different balance.
This testing can hopefully convince that it is not the board that is a problem.
For the tuning of your sails, I don't know them specific. But it sounds like you did all the right things (less downhaul, boom down, mast track slightly forward, OK fin size, longer harness lines etc). - maybe less outhaul?
What boom are you using?