Thanks Jason for your feed-back, very relevant, much appreciated.
I have indeed planned a little trap door to go from the box to the driving cab.
As for the type of driving I intend to do, let me try to explain, but I don't have off-road experience myself, so I can only relate to what I've learned from people sharing their overlanding adventures.
I don't need extreme off-roading capabilities, like rock crawling or steep incline like 40+ degrees. (i can probably write 30+ degrees here)
But I moved from my 2wd Sprinter to the Pinz to be able to escape the "crowd" when driving into CE Asia, Africa, Americas.
So going into rough roads, pot holes, passing over rocks, mud, sand, crossing small rivers,
so I can end up on my own on the other side. Where the regular cars/vans/campers can't go (or get stuck

)
I'm planning to take off-road driving lessons when my build is almost done.
So I'm trying to keep my build as light as possible ( i should finish around max 2'800 kgs) and keep the center of mass as low as possible, but still keeping some "habitability" as I will live in there all year.
The box itself, although of sizable volume for a Pinz, is quite light. 69 kgs only for all the panels including the rear one.
And not much weight on the roof, flexible solar panels (16 kgs for all of them) and a Maxxair fan (6 kgs). That's it, no rack, no awning, no storage box.
Inside the box, the top half of the box won't have any heavy storage box or equipment. Maybe some bags with spare clothes, towels,...
The heavy items will be:
-80 kgs jerry cans -> outside under the frame below the seating level
-grey water tank -> under the box at the back
-lithium batteries -> 38 kgs on the floor next to the gearbox access trap
-toolbox: on the floor at the back of the box, under the toilet
Now going one level up:
-inverter, solar charger,.. 20kgs -> on the seating level inside the box behind the drivers cabin
-fridge 17kgs empty -> on the seating level
-water tank : 100 kgs, on the seating level behind the drivers cab
-water heater, pump,.. 20 kgs on the seating level,
then the rest is storage cabinets on the seating level.
So, yeah, I would think that my build will behave like a Pinz with 6 big guys seating at the back, not so much in total weight, but in center of mass. I think.
I was told to move my water tank under the truck to lower the center of mass, but I really want to keep it outside the frozen volume. I will live in there all year and go up to 4-5'000m where it is freezing.
If you see other opportunities to lower my center of mass, let me know.
If not enough storage inside the Pinz, I'm planning to add 1 or 2 storage box on the roof of the cab, but not final yet. That certainly brings the center of mass up.