It's an enclosed cargo trailer with the same width as the Highlander, slight V-front, and 30" taller. Not rounded corners.
Could you explain more how the car behaved towing the u-haul at 70 mph? It wanted to drive in the second to final gear continuously, or kept downshifting and upshifting throughout the ride?
Also what Highlander are you referring to specifically, the 2020 gas model?
My van wanted to hunt above 65 mph. It would shift down then shift back up. I suppose if I had the ability to lock it in 4th gear (5 speed) maybe I could have lived with that, but remember that trailer only weighed 1,750 lbs, not 3,500. The additional weight and extra 18" of height will change the dynamic significantly. IMO the hybrid is out completely and I would seriously question whether or not the V6 is up to the task. Granted it has the advantage of 50 more HP and the shiftable 8 speed which will allow you to lock into the best gear, but I wouldn't be shocked if flat ground has you feeling like you are working the crap out of it to run 60 mph. If you do it the addition of the largest transmission fluid cooler is an absolute must.
Back when we spent a bunch of time camping I would occasionally see someone roll in with a Honda Pilot, Ridgeline or Odyssey pulling a 3,500 full height camper. Toying with the idea of upgrading the camper, but not wanting to change the tow vehicle for a hand full of weekends a year I would engage the people and ask questions. Without a single exception they all said it wasn't working and they were already shopping for a different tow vehicle. I was getting 16-18 mpg pulling my pop up at 70 mph (2,200 rpm), 23-25 mpg not towing, and they were reporting single digits with the lowest one I heard at 5.5 mpg and they were limited to 55 mph and sticking to 3rd gear so they were running 3,500 rpm to do it. Naturally the rest of the traffic is running 70 mph+ including semis and they felt like they were sitting ducks on the highway as everyone blew by.
I realize that a 2020 Highlander V6 is not a 2007 Honda Odyssey, but quite frankly the platforms aren't that different. In fact the van (the Sienna would also) has the advantage of a lower center of gravity, longer wheelbase, and wider track, all of which contribute to more stability.
PS Make sure you understand ALL of Toyota's towing specs. Being a recent Toyota buyer I am not as familiar yet with all the ins and outs, but with Honda the 3,500 lb rating is only good with a 150 lb driver and fuel. No additional passengers and no cargo. For every pound of passengers or cargo added to the vehicle the tow rating reduces by the same pound.