I agree with you 100 percent. Except I still believe all of Star Wars was his up until the point he sold it. All the choices for the overall direction. His choice to direct and write the prequels showed his weaknesses and his strengths. He was not a great director or writer anymore. Coming off American graffiti and Star Wars he was the top writer/director in Hollywood with two best director/best picture nominations in a row. He was so good at knowing what a story needed he hired a unknown in kershner. It was a stroke of genius. He could have directed himself, or hired his buddy Spielberg...but he went with kershner. And the little details he brought to the table gave empire that extra magic. Kershner made a better film then lucas or Spielberg could have done...and both were at the top of their game at the time. Lucas also gambled huge on that film and used his money to finance empire for 30 million dollars. Had empire bombed, does ILM even exist? It was a huge success and Lucas bought his freedom.
But Star Wars was all his, warts and magic moments. All his decisions in the first trilogy were great. A walking upright dog, a moving trash can, the tin man from oz, using the muppets to create the magical yoda, his casting decisions. All worked.
The "I know" quote from empire, they filmed both scenes of Han saying the famous quote and Han saying " I love you". Kershner said the I know quote fit Han and Lucas was unsure so they tested it on an audience. The audience loved Han saying, "I know". The crowd went nuts....Lucas looked over to kershner and said you were right, it stays. And you are right, it was Fords Idea...just like it was Fords idea to just shoot the sword fighter in Raiders when they had this elaborate fight scene.
In the prequels it was just the opposite, most of his ideas just didn't play that we'll on all aspects of filmmaking.
I think its the difference of being a hungry director wanting to see your dream come to life as opposed to an older man who has everything. The hunger wasn't there and it showed in the prequels.