Valyria being very much a slave society, how is it that the Targaryens did not impose that institution on Westeros upon conquest? It certainly wouldn’t be the only westerosi taboo that they would have broken….

Good question!

I would imagine the reason would be that the Targaryen supply was massively disrupted by the Doom of Valyria and the Century of Blood, that there was no demand in Westeros, and they were occupying a relatively small island which doesn’t lend itself to latifundi-style slavery.