I like your idea, Bastian.
I just bought a 2020 FTC PW. I'm not ready to make permanent mods to it yet. I'd like to add a removable sleeping platform that sits atop The folded-down 2nd row and 3rd row seats. Maybe a plywood panel sitting on a 2x4 frame. Easily removable. Not bolted in.
Then on top of the plywood, I could place a 5" thick foam mattress.
And I would just use this configuration for weekend camping trips. After camping, I would remove and store the platform and mattress in my garage. Then the car reverts to a 6-passenger daily driver.
I'm going to first try what the other gentleman said, just folding the seats down, putting a camping air mattress on top, then my sleeping bag, and see how comfortable it is. That sounds like a really simple test.
When I read about people sleeping in their FTCs, many talk about condensation. Some leave the window open a bit. Some use little electric fans. The most serious actually cut a hole in the roof and install an electric fan. It will be interesting to see if I experience condensation after sleeping overnight in the car.