I don’t think you’re doing anything wrong or missing anything crucial. There is something definitely screwy with the suspension load calculations. On several vehicles I’ve made, I either have to turn them into monster trucks or raise the spring stiffness to spinal compression injury levels to be able to even get a load capacity approaching an actual car.
On the 1950s not-BMC body that I had the issue with NaN cost, with 264 mm of suspension travel and a 5.00 rear spring stiffness I still only managed 200kg load capacity and that was with a solid axle coil suspension.
EDIT: Just to clarify, the Normal preset uses a 1.80 rear spring stiffness albeit with slightly more travel but even so. Getting realistic rear loads just seems to be excessively at odds with ride comfort and drivability.