Kevin, this is so great to hear! I’m tracking with your questions above as well. This certainly does happen that a client doesn’t provide a ton of detail. What I have noticed is that the more familiar you become with the themes, the more you can see in the stories, even without much detail.
What I did early on is have the client report with the themes pulled up to reference and read through as I’m working so I can review the definitions and descriptions. This helps me to see connections. I’m also trying to step back and see the beginning, middle, and end of the persons story. How did they get involved, what are they aiming for. Look for chronology; and don’t overthink it.
As you get more repetitions in, you will become more confident with your flow hypothesis. Remember, and I do believe this; you can’t be wrong. You are simply describing what is most satisfying to the person and so they will resonate with it.