I agree with Ken.
On an Isonic (any size > than 90 liters) when you crank upwind, you actually "pull up" a bit with your front foot (so no weight, only pressure up against the footstrap).
On a beam reach, you can lay off the fin and you end up with some weight on the front foot as Ken suggests.
Downwind, you have to kinda "push" the nose off the wind, so yes, you get a little front foot pressure.
How to get more front foot pressure?
It sounds like you've covered most of the comon suggestions on changing the weight distribution on your feet.
Where do you run the mast foot? ..... do your boards plane really free?
It could be your stance.... but then again maybe not. Best way to check that is to have someone take photos or video of you sailing so you can see what your stance really looks like, vs what you think you are doing.
Keep making small adjustments and you should be able to find the balance you need.
Larger adjustments sometimes pass right over the "sweet spots".
Do you have "on the fly" adjustable harness lines?
I've found having them adjustable really helps when doing the sort of tuning you are after.
Hope this helps,