A simple change to the registration fee that scales it up proportionate to Vehicle weight would be a perfect fix for this. They could even scale it down for ebikes / motorcycles and create a much simpler system for evaluating how rego is charged?
NSW already does this and it doesn't seem to have reduced the rise of larger vehicles, though admittedly the price difference isn't massive for most cars (e.g. a Camry would fall into the $330 category while a Hilux would be $505).