Hi Sauropod,
Welcome to the forum and we all started out as "Lurkers".
The Expedition is a very capable vehicle, but if you like your Flex I would look into upgrading it's capability first before trading it or your TM. You should do some research on the rear suspension and see what kind of upgrade you could do. Adjustable shocks or airbags come to mind. Might not be enough, but combined with a heavier duty WDH, could get you to your weight capacity. That certainly sounds less expensive than upgrading a tow vehicle or down grading your trailer IMHO
Good luck and keep us posted