Your E450 WILL tow 5000-lb. It'll tow 3500-lb more easily. My take would be find the lightest 3-row vehicle you like and look up its towability. It may well not be flat (4-down) towable but very likely will be dolly-towable if it's only Front Wheel Drive.
Our previous owners towed on a dolly. First a Honda, I think it was an Accord. Then a Korean Crossover. I forget if it was Hyundai or Kia. FWD "minivan" and it worked out OK for them.
An outfit called REMCO (remcotowing.com) sells "lube pumps" that render certain automatic transmissions towable. A number of us here have used them successfully. Personally, I want a toad that's rated by the manufacturer as towable. Warranty coverage is knotty enough as it is without REMCO and Manufacturer pointing fingers at each other.