First off, I don't see where you're deriving this point from. Law moving on and him being the one to land the final blow do not contradict each other seeing as they both fought him at one point and Law landed a lethal blow; what actually prevented Law from doing anything more is him getting his arm ripped off, he simply wasn't made strong enough to avenge Corazon, but he allied with the man who could. We even have Law stating that he'd done everything he could and now puts his faith in the strength of Luffy, stating that everything he does until he dies represents what Corazon achieved and that the Strawhats are miracle workers that will definitely take down Doflamingo. At no point was it ever hinted that Law was trying to move on but rather he put his trust in someone that can do what he couldn't.
And since we're talking about themes, we have a depiction of the Naval Admirals as one of the forces Luffy himself needs to surpass to reach his dreams. Luffy's scar still aches at the mention of Akainu and it's something he'll carry on his chest for the rest of his life . The scar Sabo acquired was by the hands of a Noble thus he'll be the one to destroy them while Luffys scar came from Akainu and he'll destroy him. That fits thematically.
A major theme of the Dressrosa arc was that revenge is destructive.
Kyros spent his entire life atoning for two murders he committed as a teen out of revenge. They had taken the life of Kyros' friend, and so he murdered them out of rage. He spent the rest of his life regretting what he did to them. Likewise, a major part of Rebecca's character is that she chooses to honor her mother's memory by following her beliefs of pacfisim instead of trying to avenge her. Towards the end of the arc when Kyros is fighting Diamante, he
isn't motivated by revenge and instead is fighting to protect his daughter.
Doflamingo's main motivation is that he wants to take revenge on the Celestial Dragons that screwed him over his entire life by enabling Kaido to start a war that would end with the World Government getting toppled. The world treated Doflamingo like shit, and so he wanted to watch the world burn for the sake of vengeance.
The fact that Law, who was motivated by vengeance, never got to defeat Doflamingo fits the themes of the arc. It was never going to satisfy him, and the fact that Luffy was the one to land the final hit on Doflamingo ironically made it easier for Law to move on.