With a family of 4, and bikes etc for the kids I think your truck will probably run out of payload capacity long before you hit your max tow weight.
A Popup, or Hybrid will get you lots more space with less weight. We had a popup for 9 years when our kids were younger. It was great for us. We pulled it with a Honda Pilot, or my truck depending on what all toys we took. The pilot pulled it really well.
The biggest problem I could see with a hybrid, or popup for you would be with living in Seattle, I assume it would be a pain to get the canvas dry to prevent mold and mildew.
