Is that a hollowed-out tailgate on an SUV? Wow that is an impressive idea!
OP, your distinction of "Overlanding" and "Offroading" is certainly valid!
We have recently begun using our 4Runner for this very purpose. Funny enough, we drove 500 miles straight from the dealership for a weekend of camping! Since we carry the dogs in the cargo area, we were super limited on storage on a subsequent trip when our friends were in the car so I just bought a small roof rack from Walmart and made some roof bars out of steel C-channel to mount it lower. It was cheap, but it is light enough for me to take on and off myself and with a few mods (like some extension) will be plenty for what we need. I had co sidered buying a big fancy one, or even buikding one myself, but given how nice it is to be able to remove it for the wind noise and mileage when we don't need it, we might try this out for a while and see if it is big enough!
Probably the next big challenge for us is water storage. I have several 7gal water kegs for emergencies and camping (of course, only 1 is plenty for 2 or 3days), but they are obviously a pain to get up there and I dont love having all that weight up top. Though, in the grand scheme, probably the next best options are either a custom flat water tank that sits across the cargo area under the dogs' pad, or a $2,000 rear bumper with swing-out carriers. So, I will probably alsl make-do with the water on the roof for a while lol.
And of course, good tires (another set) and under-armor and a mild lift are coming soon.
Hope you get some good answers!