If a user reads only the first screenshot and can't say what the app does in one sentence, you have a conversion bug, not a marketing bug. Format that works: big overlay text describing the value ("Track your training, no spreadsheet needed"), small phone mockup behind it. Most apps make the phone the hero. The text is the hero.
Rewrite my App Store screenshot 1 so a user who reads only that one screen can say what [app] does in a sentence. Make the value-prop text the hero, like a big overlay reading 'Track your training, no spreadsheet needed', with a smaller phone mockup behind it. Give me 3 overlay headline options.