a What if? If the Targaryens had another Valyrian noble house they brought over from Essos and needed to reward them with lands and castle for their help in the Conquest; Orys Baratheon got Storm's End, would it make sense if they were given Highgarden/The Reach?
If there were another Valyrian noble house at the Conquest that had come over with House Targaryen from Essos before the Doom, it would have been on Dragonstone for 100 years already without lands and a castle. That doesn’t sound especially noble, y’know? So therefore extremely unlikely. House Velaryon and House Celtigar were Valyrian (non-dragonlord) houses that had left Valyria before the Doom (probably around the same time the colony of Dragonstone was founded or some time in the 2 centuries between that and the Doom) and they each had whole islands, complete with castles. And they were allies of House Targaryen in the Conquest, without promise of further lands.
Alternatively, this is the exact situation of House Baratheon (presupposing that Orys was not Aegon’s bastard half-brother as rumored, or that his mother was from a landless Valyrian family of Dragonstone). So no need to make up a what if when Orys and Storm’s End exists already. Hope that helps, I guess!