I ran into that issue too w/ sentence-based flashcards, where I almost immediately memorize the sentence itself and the whole thing becomes self-defeating. Similarly, I thought to use LLMs to generate fresh sentences on-the-fly, but the output was never reliable enough for my use-case... e.g. I came across some grammatical construction that the LLMs refused to use correctly. But, this probably depends on the language in question, since it sounds like you had success with it! Maybe I should try again at some point, now that the consumer models are a lot beefier than they were even six months ago.