Oh, you mean Daeron I. Interesting. Part of the issue there is that Aegon’s First Dornish War had some really profound path dependency issues – the Dornish aren’t going to forget their kingdom ravaged by dragonfire, the Reachermen aren’t going to forget their kinsmen slaughtered in the sands or the raid on Oldtown, the Stormlanders aren’t going to forget their kinsmen mutilated after a prisoner exchange or the burning of the rainwood.
Also, Daeron I desperately needed a big military project to unite the realm and restore the authority of the monarchy after the Dance and the False Spring. While he’d probably have an easier time selling such a pact than Baelor and Daeron II did, thanks to not being murdered under a peace banner, bringing Dorne into the realm peaceably doesn’t quite solve the problem.
It’s actually quite reminiscent of Edward III’s position when he took the throne of England – recent civil wars, a king deposed and then murdered (although for Aegon II they didn’t bother with the deposing), defeats abroad, a long period of regency, etc. Hence the idea of claiming the Throne of France as a way to get the barons on-side with the lure of French territories.
So maybe Daeron I could have looked east? Stepstones are a bit meh, but maybe a holy war to reclaim Andalos for the Faith of the Seven? That might have done the trick.