They then have a progressive structure that convinces you new epiphanies are just around the corner, you just need to put in a little more effort / time / cash--but there is no epiphany waiting that will be as useful as the original epiphanies.

I think "epiphany" isn't a good way to think about scientology. The advantages you get by not being emotionally reactive to triggers, aren't about "epiphanies".

This happens lots of places. I recall reading about some Alexander Technique expert, who continued doing lessons in the hopes of recapturing the first moment when he experienced lightness in his body. He never could, because the thing that was shocking about the first time was the surprise, not the lightness, and no matter how light he got, he could not become surprised by it.

Could you source that story? To me that sound like someone not practicing "beginners mind" and as a result getting things wrong.

But here we run into the issue that, while rationality may be the common interest of many causes, the "something new" is unlikely to be a specifically rationality thing.

Eliezer mostly wrote about his own thoughts on rationality and at the beginning of LW, I think there reason to assume that it covers everything meaningful there to say about rationality.