Bone Jarring Ride

Most single cab utes that are built to carry a tonne or more of load ride hard when empty as the suspension hardly works.
Until you fit the canopy and other accessories I would put some weight on the tray/tub as that will take the edge off the harshness. As you have found 200kg on the ball isn't enough to soften it but might even be having a pendulum affect and exaggerating the ride.
400-500kg on the tray, sandbags, or something similar will take the harshness out.
Once you have the canopy and other gear onboard and if it still rides too hard then you can visit a suspension place and see what can be done.

Craig.
Before you consider suspension, perhaps think, are the tyres heavier construction for a single cab vehicle designed to carry weight? and heavier duty than you require!
Are the sidewalls more plies which makes them stiffer and less absorbing of road conditions.
What tyre pressures are you running??????? If there isn't much load, the tyre pressure should be adjusted to suit the situation while retaining sufficient pressure for correct control aspects.
A high tyre pressure above what is needed will make it ride hard. Silly Mercedes Benz, fit RUN FLAT tyres to some of their vehicles, No Spare, and they ride hard, having much heavier sidewalls. but normal pressure.

Because the tyres ARE the PRIMARY SUSPENSION of a vehicle, appropriate tyre pressures to suit the use and load should be the first consideration.

IF and only IF, your vehicle was actually received a pre delivery inspection, the Bunny doing the task may have made the tyre pressures capable to carry it's full load immediately, because that is what it is sold for.

If your vehicle is towing with a 200kg ball weight, lever action around the rear axle by van ball weight means some of the original weight is removed from the front wheels and suspension. If the tyre pressures are far too much, then every bump will be felt as a personal assault because the rock hard front tyres cannot absorb the instantaneous road shock and they will try and throw the forces to the front control arms and shock absorbers, which may not be the best anyway, even when NEW
Checking and ensuring tyre pressures are appropriate is better than dental bills.
Understanding your vehicles requirements is something which you are responsible for, to learn and apply as required.

Is the ride in the ute ok without the van? If it is, the issue might be the ball weight.

Manufacturers will provide a a ball weight at tare i.e. unloaded. If you fill the water tanks and pack the van, ball weights can increase dramatically depending on the location of the water tanks e.g. 200 kg at tare might become 400kg when loaded. When hooked up to the van, is there a gap in the utes rear suspension above the bump stops? If it is on the bump stops, the ride will be terrible.

My single cab DMax ute rode hard until I put some weight on it. They are a work ute. My last DMax was a twin cab and I had to upgrade the rear suspension to carry the same load as it would bottom the suspension on large bumps

Tighten bra straps and remove dentures.

Well you have set the cat amongst the pigeons with this post.
The basic fact of the matter is the DMax is a ute, it's a workhorse and the ride in them can be stiff and unpleasant..especially when there is nothing in the tub.
In your case with a ball weight of 200kg, which is getting up there, towing around a van nearing 2000kg hanging off the back of said DMax..the standard OEM rear suspension would be working overtime.
The good news is that your are starting with a complete blank canvass, the not so good news, is that get a decent ride out of the DMax with your 2000kg is going to cost a few $$$$ more.
Now I'm going to come at this from a different angle.
It is now the best time to get a certified mobile weighing mob to come and go over your combination and give you a written report..it's about $250.00 or there abouts.
They can impart information on how to go about solving your particular issue..a good reputable mob has seen this issue time and time again..