If there was any way to keep coral sand, oyster shell grit or limestone sand or fine gravel in larger quantities in the tank or filter, not let it blend with the substrate, I think that would be the best solution. Perhaps a second filter with a lot more of these materials. I am already running 2 filters on the 88 gallon tank, so I could sure put more of something in them, even if I have to reduce the amount of sponges in them.
I also live in Seattle where we have soft water with low KH. I'm using the aquariumplants.com substrate in the dark gray color, which is close to the SMS charcoal.
I put some aragonite sand that I had left over from my SW tank into a filter bag and put it into my canister filter, and it works perfectly to keep the KH up. It doesn't take a lot, really. I think the water constantly flowing through it in the filter makes up for the small quantity of sand.
Hope this helps!