I'm quite sure that railway modellers have many collectors in their midst. I know of at least one more.
Because they are relatively small, road vehicles in 4mm scale don't need too much work to improve their appearance as working examples. As you observe, a layer of matt varnish is the easiest improvement and the rest is down to small quantities of whatever material you choose to use.
I considered it - at least with one of the lighter plastic buses - but there's no really straightforward way to get a circuit of road into
the layout without a lot of work!