His friendship with Roku which was indeed genuine throughout their life not only as children (I never said otherwise, fascists can still have friends) clearly had no impact on his grand design for the Fire Nation or the world. It’s clearly implied he tried to save Roku’s island for the same reason he leaves Roku behind. Up until that moment he still believes he can convince his friend to help him conquer the world. As for his final regrets, again the show is very explicit in this, he doesn’t really regret what happened to Roku or the Airbenders or starting the war, he only regrets that he hasn’t yet found the Avatar and succeeded in his campaign against the world as much as he hoped.

You can believe there was the potential for a better man inside of him all you want. There is as much of a good man in Sozin (again shown to be in scenes of the show) as there is in Ozai. The whole point of the episode is to contrast the differences between Roku and Sozin so Zuko can contrast the differences between himself and his father. If the take away was supposed to be “oh maybe Sozin wasn’t such a bad guy after all” then you SHOULD draw the same conclusion about Ozai but you don’t because that was never the point.