I’m surprised that at 5000kg your player can still move it that easily. You could try setting the angular damping. It will be under the Physics category of the mesh - where you set the mass to 5000kg. Is “Enable Gravity” checked?
Just to clarify -
Simulate physics should be enabled.
Mass in KG should be CHECKED and set to desired weight
Enable gravity should be CHECKED.
Modify your damping settings to help keep it from toppling so easily.
That should do it.