But it's the same route. You have to pass by Dorne to get to the Westerlands by sea.
You're right about HighGarden, and the same should apply to Casterly Rock. Exactly how are you going to take that back easily with thousands of Unsullied inside?
And if Highgarden was so weak that it fell that easily, then why march down there at all, losing Casterly Rock and perhaps KL for lack of troops. If the Tyrells were so weak their castle fell that quickly, then they'd have been a joke as a field army, and not worth fighting at all.