This app sounds similar like a deck of Flash Cards, or Quiz cards.
With flash cards, the goal is often to work one card after another, keeping the user in a focussed state. They do a burst of exercises, then take a break. It's similar to a sprint.
Having Next and Prev links will save them from having to hop back and forth from gallery to detail.
If you give them a close link, they can always go back to the list.
Keep in-card navigation simple.
You mentioned having a dropdown to go to a specific card. This assumes they've memorized a title or sequence number. This will get difficult with 50Are you basically replicating the cards. Plus a dropdown with a long list in mobile getsa sliding drawer? That could get unwieldy, and the user is then focussing more on navigation than rapid acquisition of knowledge.
In that case, they can just close the current card and pick from the list, which you said has a preview of the exercise itself.