Sounds like you expect a lot from your equipment having a G-Wagon
Let's explore the boxes you are requiring
Hardshell - quite a few choices in styles and materials. The lighter the shells, generally the higher the price. The ones coming from China tend to weigh more
Fully Open - I take it you are also looking at a Columbus style tent that is hinged at one end. My wife really prefers them as it is easier to change clothes. Better for warmer weather, some really open up a lot. Check mattress thickness because in warm weather you don't need as thick of a mattress and manufacturers can save weight and cost. Open and close in a heartbeat, and very convenient. The Maggiolina style, rises on both ends tend to be better for cold and very cold temperatures. Built more like a mountaineering tent. Some manufacturers offer higher roofs so you can store more bedding inside and have just that little more headroom, again important in cold weather. They are generally designed to be short to minimize internal volume, again a cold weather thing. You have to special order XL sizes.
Able to fit an annexs - This one is a question mark. You are sending out a mixed message as the hard-shells are designed to be quick and easy and annexes are not. There are annexes available but I'm not a big fan of them. The Verandah style, as shown in your link, are designed as place markers when you are camping in a park. Something of a knock off of a European design, and they use their tents differently by setting up the Verandah at their campsite and using it to store their gear. They will leave them up all summer, like a second home. Heavy and cumbersome. They do make more simple designs that use bars attached to the base of the tents that you can't leave. These are much more simple and fast to set up but you really need a sail or awning maker to adapt to fit your use. I have a custom built awning for mine designed for more temperate but really wet conditions.
XL for taller people - Think of your tent more like a sleeping bag. I generally go up there to sleep, and they aren't the best for hanging out in during the day. If I bring my wife and she is going to stay at base camp, I bring a heavy duty easy up for day shelter. Again, it all depends on the type of camping and conditions you go out in. If you are going out in temperatures below 30 degrees, especially if you are by yourself, then a smaller tent makes sense. I had a Maggiolina Alpine, back in the '90's, that was designed for Switzerland and Germany, and was really good for winter camping as it was especially cold. Once buttoned up, it was surprisingly warm.
2 Adults and small kids - here you also have to look at your rig. G-Wagons generally fit a medium sized tent which is good for two adults on a long journey or a couple adults and a small child or two in a pinch. The large tents start to get heavy and are really cumbersome. If you are going to fit one on your top full time, it might mess with your fuel mileage. They get a funny "end plate" effect if they hang out over the sides of your roof which really disturb the airflow. A medium tent might actually improve your aerodynamics and mileage. I'm also not a big fan of tents that weigh more than 135 pounds as they are difficult to get on and off and start to affect the vehicle's handling. That is just me, there is an ongoing debate on this.
Hope this give you more of a user's critique rather than a manufacturer's hype. We have crossed the country many times and have found our hard shelled tents invaluable and improve the experience. If I was buying a tent for a couple weekends a year, I'd buy a nice ground tent, yes, I have two ground tents and like them a lot.
