Ground camping SUCKS! In the dirt, mud, sand. There isn't a way to keep everything really clean because you are in the dirt. Backing up the jeep to the tent works great if everything is dry and clean, but leaving doors open and creating makeshift hallways will let water into the tent, now everything is wet. Ground camping is great if you are hiking and need a light tent for a few day hike. But to live out of for a little while, they can really suck. Especially if your goal is to get out and see different places. Yes, this is my opinion, based on where I live and what makes me comfortable, so take it with a grain of salt.
The rooftop tent is an awesome option. Not one I prefer only because I built my jeep to drive and explore OHV trails. You can do this with a rooftop tent but not as well with the seriously raised center of gravity, plus you have to break down camp to go drive, if you were planning on leaving that day or not (yay for setting camp backup). But if you are looking to drive somewhere and hang out for a few days, I wouldn't overlook this option. Get it with an annex (that hangs down to the ground and encloses the space below the tent and you will have more dry space than you could ever use in bad weather, while also keeping setup simple and NOT using the annex when the weather is good.
I personally wouldn't write off the trailer suggestion. The speed limit would not be a problem for any trailer suggested to fulfill your mentioned needs. I personally, recently picked up a Fleetwood Evolution E1 popup trailer myself. My 4.7l Grand Cherokee has no problem pulling it the speed limit at probably 3500lbs loaded. I don't live in a flat state either... just driving 10 minutes to work I see more elevation change than some entire states have. I have a young family and a dry, warm hangout space really is important in the wet pacific nw. WAY overkill for what you are trying to do and NOT what you are looking for, I do realize that. You could pick up a smaller enclosed trailer for a couple grand (as suggested earlier in the post) and it would tow like its not even there and give you a pre-setup tent that you just open the door and climb into.
Another option would be to put the rooftop tent on top of a simple utility trailer. Trailer weight is kept to a minimum. Jeep center of gravity doesn't change. and you have tons of storage below the tent on the trailer.
The reality is, people spend tens of thousands of dollars on overland setups because there is always a new solution to a problem, there is always some piece of gear, some tent, some awning, some stove, lighting, trailer, tent, etc. that makes life easier. So there is a LOT to weigh in all this. Going budget friendly has its MAJOR tradeoffs when it comes to living out of a vehicle, espcially a small Jeep Grand Cherokee.
Also, just wanted to mention from a much earlier post. If you were to remove the passenger seat. You will need to get a resistor for the pretensioner and side airbags in the seat. Having those unplugged will disable ALL airbags so you will want to trick the computer into thinking they are still there.
The interest in the hitch mounted platform.. they work great but also have a HUGE impact on the rear suspension. Just putting that gear in the back of the jeep would make it squat some, then push it back 2-3 feet away from the rear axle and you are giving all that weight more leverage. You will drive around squatted. Driving at night would be blinding oncoming drivers. Simple, functional solution, that causes other problems.
I also like your rack you made in the back. My only concern with it is how much space it takes up. With respect to the size of the tubing used and the space around the system. I realize you may have loose things to stuff on the sides but its really not super usable space on each side. So its eating into already limited area. I would personally plan to have the cooler/fridge on the floor in the back and build a smaller aluminum (or lighter, smaller, thinner wall steel) structure to support storage of other, lighter things. Just make sure to back the shelves into the cabin so if you are in a collision you don't have Rubbermaids assaulting you from the back of the jeep. Then buy a slide for the fridge and you are set.
EDIT: I am seeing the rack in the back runs all the way to the front, which I didnt notice before. This would give it more functionality than I initially thought, but still not a very efficient use of the space. How is access to the front of the rack from the side doors?
All my .02 cents. Your mileage may vary. Have fun with it!