3.5t vans payloads are rubbish unless they’re low roof/swb models. Most companies go for LWB/maxi bodies for flexibility and once you add roofracks/ramps/lifts/air con etc the payload shrinks.
Our LWB sprinters with rack and ramp can only take 1025kg and are really easy to overload.
And remember the axle loadings as well - which means that any max load cant be evenly distributed its got to mostly be over the back axle.