How would you rate Daeron I had he actually succeeded long-term in his conquest of Dorne? Also, what do you think he would have done (apart from raze Dorne to the ground) had his murder been thwarted considering he was only eighteen at the time?

To me, the big problem with Daeron I’s conquest is the fact that, after he defeated the conventional forces of Dorne and left in 159 AC, the country rose up in rebellion, leading to a bloody insurgency and counter-insurgency that killed many times more people than the war did. Even if he had avoided assassination, it would have kept going until Westeros finally revolted against the sheer cost of lives or he was finally pushed into a horrific Carthaginian solution, rendering the entire affair pointless. 

So in that way, Daeron I was kind of lucky that he died when he did – he got to go out as a Medieval James Dean, forever young and beautiful and on the cusp of another impossible victory.