I have been thinking about learning apps for years now (without actually trying my hands at the problem) and my idea for solving this is dropping cards, and instead use a “knowledge graph” and its nodes as the primitives. Then you can have the vocabulary and for example example sentences with a missing word. If that example sentence contains a word you have also in your database, answering this harder question will also count as a review for the used words, so you won’t have to review them separately anymore, while you still get to practice them at no additional effort.