As I understand it, as well as a horizontal towing limit (half the car’s kerb weight or as defined above) most vehicles also have a maximum vertical load for the tow bar. I believe this is to avoid overloading the car behind the rear axle (rather than the physical limits of the tow ball). In my experience this is typically about 70-80 Kgs but may well be less for a light vehicle. The maximum vertical load is usually defined and written in the cars documentation. I have no idea what happens to your insurance or leagl liability if you exceed this limit – but I am sure that in today’s world it would give the insurance company wriggle room.
Heavy loads in the car should be stowed between the axles and well secured (put the rear seats down if necessary) and then the trailer should be easily lift-able onto the tow bar. Otherwise a heavy trailer nose weight combined with heavy items on or behind the rear axle will give you the bad to dangerous handling characteristics described above.