1. Taxes on ICE vehicles don’t begin to capture the cost externalities associated with their use. Health costs from tailpipe pollutants alone are really significant. It was never a hypothecated tax because (a) the UK doesn’t really do any hypothecated taxes, and (b) the costs aren’t just roads
2. The weight difference between an EV and an ICE vehicle is completely irrelevant for the purpose of road wear. Road wear is essentially entirely caused by large vehicles — buses, lorries, etc — because as you yourself pointed out, it grows as the fourth power. So a 1.5 ton EV supermini causes about 1.8x the damage of the ICE vehicle. But a bus weighing 12 tons causes something like 1,500 times the damage, and a 40 ton artic (= semi) causes something like 30,000x the damage. Even the difference between a heavy UK SUV at 2.5 tons and the 1.3 ton ICE supermini is just unimportant by comparison.