That 3500 lbs payload sounded like a lot, I figured you could never put that much in the truck w/o leaving the vault open, but I did the math and it came out differently than I expected. Someone wanna check my numbers?
First off, you need to subtract the driver and passenger weights from the total payload. You probably have at least one helper to load and unload, so here in the land of plenty, subtract 500 lbs for the weight of the helper and driver.
Some examples of some heavy stuff you might end up hauling:
A "yard" (27 cubic feet) of not-so-dry sand can be close to 3000 lbs. A typical Bobcat bucket capacity is about 1/2 yard so two heaping scoops of sand and your truck will be at or over weight capacity. A yard is not a very large pile. If you spread the sand pile level in the 78 x 57 Cybertruck bed, it would be less than 11 inches deep.
Or some sack-crete. A 60 lb bag is about 1/2 cubic foot. So 50 or so bags would hit the max weight capacity and take up 25 cubic feet of space. This would take slightly less space than the sand (it is mostly just sand)..
Or maybe you are hauling something less dens like retaining wall blocks. The larger ones are 77 lbs each, and are 8x12x18, or 1 cubic foot each. You could carry like 40 of them and fill about 40 cubic feet of space. Spread out in the bed, you could fit them, in 3 layers of 18 which would be 24" high. Don't know how high the tailgate is, but if its less than 24" you could start the top layer more towards the front and get the cover closed.
If you really want to haul water, you would need a tank. Unfortunately, the standard sized low profile tanks for pickups tend to be designed to fit over the wheel wells, so you lose capacity (the one catalog I am looking at, they are also 58" wide, so won't fit in the cybertruck). But a customized tank that would fit well in the Cybertruck, it could be 78" long x 57" wide and 24" high and would hold about 3000 lbs of water (375 gallons). Again, not sure how high the tailgate is, but you could design the tank to be angled if necessary so you could close the cover.
Wood would be something even less dense. 3/4" plywood weighs about 70lbs per sheet. You could carry up to 42 of those. The pile would hang out the back on the tailgate and the stack would be 32 inches high.