I ride up in the Laurentians in Quebec and have this problem too when we have deep powder .....
I think you have to start straight downwind to get board speed before using any edge, and when you do make the progression of weight gently onto your front foot so you don't "submarine"
You need a lot of power to ride in deep powder so don't be shy to use more kite power and also try and find the biggest widest snowboard you can to reduce your "footprint" on the snow.....
and get it waxed too .... Have fun.
