Great app! I've been building something similar, but for less advanced language learners, who wouldn't understand definitions in their target language.
My app [1] is basically a combination of SRS flashcards with an ebook/YouTube/Website reader. Unlike Anki though, AI creates example sentences, definitions, images and audio.
I find it interesting that you want to get inspired by Duolingo. My approach is to have the most efficient grind possible - no gamification. I've found Duolingo was wasting so much of my time with exercises that did not really teach me anything and took a long time to complete + the XP points/levels etc. were quite distracting.