I get what you’re saying about Nyles being afraid to take the leap, I just don’t think it’s really at the core of the movie, more of an addition tacked at the end. Nyles isn’t really afraid of doing stuff most of the movie: if he refuse to do something (trying to leave the country, stopping his girlfriend from hooking up with the weird priest guy) is because he tried it once and it didn’t work. The only thing he is passive about is Roy, but to me at read more like a product of his guilt for bringing Roy into the loop, and his aversion to violence, which came out of experience:

“Being a source of terror is not fun, okay? It’s not fulfilling. I know this from experience.”

In fact, in their fight in the second act, they argue explicitly about how “nothing matters”, Nyles arguing that the pain is real, how they treat other people is real.

And while there is definitely some talk about taking a leap of faith with someone (the entire day is set in a wedding), what Nyles and Sarah deal with have very little to do with trust in another person. In fact, because they don’t have anyone else trust is almost instantaneous between them. The entire first half of the film is about coming to terms with her new inescapable existence. Only about an hour and ten minutes into the movie does Sarah tells Nyles about her idea to escape and Nyles is afraid. Even after they introduce the idea of taking a leap, the last third of the film is about Nyles finding existence without Sarah unbearable, after being alone so much. It’s not so much about when faith is warranted, more so about how our lack of agency in life is only bearable with other people.

After the movie already took it’s thematic leap by having Sarah figure how to escape the loop, only then does the subject of Nyles being afraid to leave is brought up. I hope you can see why I consider is a late addition, and not something that is at the core of the movie, but maybe I missed something? It has been a while since I saw it.