Hi,

You might want to consider that 1000 pounds of weight is half a ton. Most pickup trucks with a frame are built for that weight. Are to driving on back roads of in some state that just has bad roads. 🤔

If you look on the door tags along with tire pressures you can see what the Gross Vehicle Weight can be.
You subtract the nominal curb weight from that and you find the vehicles capacity for cargo.
This is usually a calculated figure used by manufacturers by assigning 150 lbs. to each passenger.
You do the math tine four or five passengers.
Then you could add an allowance for luggage. You then see why the trunks on cars shrunk or the SUV idea gets crossed up. Maybe that when the marketing called it correctly as being a “crossover.”
Not really excellent for either job but a compromise.

Considering the age of your vehicle I think you are easily exceeding the quality of aftermarket parts.
Especially if you want to retrofit with Volvo minded limits under there. Nivomats really sucked on the 700 series with only everyday usages besides their prices. A Volvo big mistake.

Road dampeners do not increase carrying capacity one bit. They are made to limit the sprung weight loads excessive movements. The heavier springs are wearing out the dampers period. A Vicious circle.

When carrying or moving any mass you are messing with the dynamics of physical motion.
This means, to some extent, an exponentially greater factor of overloading.
You cannot judge a static (sitting) view of the vehicle by looking capable with the seats down.
Any additional loading has to be distributed between the axles.

Apparently the back end is not capable. Any load past the centerline above of axles, front or rear, compounds itself on a suspension.
Too much in the rear of the vehicle affects the drivability of control of the forward motion of a vehicle.
The front end can even become lighter of its designed loading.
Like the allowably tongue weight of a trailer shows up very drastically on a vehicle. Headlight alignment goes to pot too.

You would be better off using a trailer and put that weight over its own a separate axle.
You then will easily see that there was many limiting factors as I suggested.

Phil