The truth is, an epistolary format may not work well for an entire thriller. It's great for certain purposes--I made great use of diaries and journals in my Virginia Reed books. In fact, they probably wouldn't have worked without them. But only within the overall narrative. Not as the narrative itself.

Anyway, outside that boundary, I'm trying to make it work. I'm installing tons of transitions, for instance. I've had the main character state in the third chapter that the following chronicle is based on Witness Statements, Trial transcripts, and so on. I believe the first 5 chapters work, especially the first chapter. The second chapter is written in a Witness Statement format, including the interrogators questions. After that, with explanation, the stories are told in 1st person but without interruption.

I'm hoping that works.

The next ten chapters will tell the tale. After that, the story is fine. But until I reach the spot where I started putting narrative into the accounts, it's going to be tricky.