Bottom line...find out what the dust is, if you haven't done so already, before going any further.
So far the attack angle has been from an infiltration standpoint, which is a reasonable approach...even more reasonable if the infiltration can be determined to be the contributor to the excessive dust problem. However, since it appears you've gone to extraordinary lengths to seal things up and you aren't reporting any diminishment in dust accumulation levels, maybe the source is right in the home vs. coming in from outside. A painted surface flaking off, for instance.
It'll cost a bit to find out what the dust is, but would give you some idea as to its source, IMO, and how to mitigate it. The way things are going now, you could end up with a very tight house, which is great for holding in heated or cooled air, but with the dust generation source unchecked, could actually be worse than if the house was more leaky.