Admittedly, those are already handled by something else where I live: A regular class B license only lets you drive vehicles with a max weight of up to 3.5 metric tons; everything beyond that requires some kind of appeal l special license and those cost a couple thousand bucks a pop and have additional strings attached. Not something you get casually.

So I would handle commercial vehicles like freight trucks, buses, tractors etc. differently from regular cars. The use case there is usually that someone has a specific professional need for such a heavy vehicle and not that they want to drive their kids to soccer practice in an Actros.

If your country doesn't have truck licenses and random jackasses do drive around in semi trucks for fun you can just go with a tiered approach. I'll use the European definitions of vehicle classes here; adapt as necessary.

Commercial vehicles get taxed by maximum weight. That includes everything designed to pull a semi-trailer, tractors, self-driving farming equipment, everything capable of seating more than eight people, and everything with a max load above 3.5 tons.

Personal vehicles (ie. everything that don't match the above criteria) get taxed by empty weight.

So if someone decides they want to drive around in a Peterbilt with no trailer attached they still get taxed as if they were hauling heavy loads all day because that's what semi trucks usually do. But someone in a regular car gets taxed as if their car was mostly empty because that's usually the case there.