Manga

I'm not sure I can agree with the idea that the point of that was the action... But rather the mystery of what those spirits were. Same as any horror Miko encounters in the series.

You're attributing your loss of interest to a genre shift I don't think was there. The hooks of intrigue simply didn't interest you, which is absolutely fair. But they are definitely the reason the rest of us are still reading, rather than the "action".

Although, it does help that the art has been consistently amazing.

IMO the series has maintained its quality, and even improved over time, such as by dropping the fanservice.

I suspect it might not've been your type of thing to begin with, and the main reason you enjoyed it at first was the novelty.

The series has done pretty much everything it could have done to stay interesting within the premise, I think.