Love the UI. Do consider adding product walkthrough for guest account. I couldn't make much sense of the purpose of the product or navigation. May be anki users will get it better though.
btw, are you using antd? or any specific ui library?
covered before: https://news.ycombinator.com/item?id=20029466
anki could use a facelift
70 bucks a year for flashcards? No.
Unlike Anki, I believe Mochi is proprietary.