I think both Johnson and Lucas overthought it.
Having been on board since 1977, I don’t think Lucas started to give it any thought at all until after the single small story he had written started to make tons of money at the box office. Honestly, he didn’t even have a sequel idea, let alone some deeply thought out mythology.
Johnson just seemed bent on the angle of “I’m going to shake up the Star Wars franchise and turn it on it’s ear”. Where did he get off deciding the franchise needed shaking up? What it needs are great characters and stories.