What I don’t get about Dany’s strategy in GOT is why didn’t she land in Dorne instead of Dragonstone? Wouldn’t it have it been easier to gather all of her Tyrell and Dornish armies together and attack King’s Landing from the South?

Dany landed on Dragonstone because that’s where she was born. It’s not so much about strategy as dramatic parallels. 

But a Dornish landing wouldn’t be easier, necessarily. Gives enemy a bunch of time to prepare as you march up from Dorne through the Stormlands or through the Reach, and you’re potentially still stuck south of the Blackwater Rush as Stannis was, although you’ve got dragons which helps.