Seat cushions can greatly help against numb bums but if you have the budget why not change your seats into better ones. Either way it is worth the investment
A faulty wiring or a 12v battery issue might be the issue at hand. But an OBD Scanner might give you a better assessment on what the problem really is.
My friend has this kind of setup on his Kia EV6, let me get back on you. I'll provide the part number, Kia usually has parts just go ahead and give them a call.
A stealth hitch and Thule Aero is a good choice as well. A Saris Bones rack could also be one of your choices and all of these are available on Amazon.
There are rust removing sprays available in shops around your area, rust usually shows up when a part gets soaked or had contact with water. How about using WD-40 to try and remove that rust?