Well, TROS and TLJ are fundamentally incompatible, so you shouldn’t take Luke’s later “I was wrong” to heart if we’re talking about how TLJ discusses a middle path.

To me, the films maintain that the dark side is rooted in selfishness. Not anger, hatred, fear, jealousy, etc. alone… Those are all just extensions of self-preservation and greed - manifestations of the extent you are passionate about what you desire. Being selfish is a natural human instinct, and I think ROTJ especially shows that it actually doesn’t corrupt you. Not any more than a child learning and pushing the limits of what they’re capable of, men earning money and status, or a politician with “power” in an authority sense.

Two examples of the Dark Side we see in the OT: Luke Skywalker wanted to save his friends so badly, he became angry, let his hate for the Empire drive his actions. Darth Vader once wanted to be more powerful than other Jedi, he let that jealousy dictate his path. But they were both able to pull back. They could make that choice, and Vader after thinking it was “too late for him.” Dark side corruption isn’t magic, you still have responsibility for yourself. It’s never too late.

Perhaps the idea that we should be thinking about, isn’t what would make a gray Jedi different, but that the examples we follow that have triumphed were essentially gray Jedi. TLJ advocates for a middle path by straight up contextualizing the Jedi teachings as having failed, twice, and that the future should grow beyond those teachings.

I imagine Lucas’s ST would have skipped the extra steps and gotten straight to that point. His PT already shows that the Jedi as idealized by Obi-Wan was far from perfect. Perhaps his Luke would have been enlightened on that front already, or the conflict would have been introduced in VII and not VIII.