Yes you would need the long. Or put the oldest kid 3rd row with 1/2 the bench down for gear which was a common way I did trips with a grandparent as the 3rd or 4th seat behind the front row.Yep! With the 3rd rows up in the Sequoia or Expedition, I'm guessing there would be more cargo space in your Sienna behind the 3rd row.
Roof box and 1/2 the rear stowage might be perfectly fine till you add a trailer. The standard length full sized SUVs don’t drive big especially the new ones with their tight turning ability and 360 cameras. The Expedition has been great it replaced my daily Subaru Outback and my kid hauler Sequoia it was a 2 for one move on my part. The Expedition crushes the Sequoia on mileage and even beats the subaru on loaded trailer camping trips. Its been a great decision so far.