Do you think it’s possible that Aegon joined the war against Volantis as a quid-pro-quo so the other Free Cities wouldn’t intervene in his conquest of Westeros?

Eh, the other Free Cities had their hands full trying to fight off Volantis.

I think it was more that Aegon didn’t want to have to deal with a powerful, united Volantine Empire across the Narrow Sea when his conquest started.