Review flash cards by frequency.
When reviewing flashcards, it would be great to filter them by frequency. One category could be the "1000 most frequent words" followed by a "1k-3k most frequent words" and so on.
Doing so may allow users to not be so overwhelmed by the number of flashcards. If you tell yourself, I'm going to stay on top of the 5000 most frequent words, then you can ignore the words that are less frequent and not by stressed about letting them pile up, because you are not going to see them often in your text.