My problem was, and this is obviously big spoilers here, that I quickly guessed that he would be the insane one. And the whole movie I was just hoping that wouldn't be the case. That Scorsesee wouldn't make that film. I thought he was going to make us think that but reverse it in the end, showing it was a conspiracy all along and surprise everyone. But that didn't happen. I found that disappointing. But after that reveal, I felt it ended well anyway.
It also occurred to me early on that this could be the case. It's hard not
to think it these days right? Like you point out, it's hardly a new twist. But for myself it was more of a passing thought and not something I hung on to. I didn't see any way to prove that theory at the time so I shelved it and busied myself with taking in the film as it was. I only revisited the idea when they got trapped in the cemetary and his partner started pointing out the "coincidences" of Teddy's situation. But even then there was no way to be certain.
It becomes more and more apparent what the truth is as things go on, and as the film nears the end it's all but certain in your mind. Even so, when he gets to the lighthouse and everything is explained to him I was not unsatisfied. Maybe my lightbulb moment wasn't as shocking as it might've been, but I still got a lot of pleasure from it. I revelled in the precision, and seeing the final pieces of the puzzle pop into place.