You should check out my other app, Nihongo (https://nihongo-app.com), it has functionality like this, and is one of the reasons I developed it in the first place.
I take DRM free ebooks and copy them into the app one chapter at a time. It automatically creates flashcards for all the words that appeared that I don't already know, and I can filter down to only words that appear at least twice.
I take DRM free ebooks and copy them into the app one chapter at a time. It automatically creates flashcards for all the words that appeared that I don't already know, and I can filter down to only words that appear at least twice.