However, I have no idea how to do "onboarding". You just copy-paste some text into the Chinese box at the top, and/or type characters into the decomposition keyboard in the bottom, or pick a topic from the heading row for some pre-prepared material.
I haven't got any users though, since 2017, so obviously I'm doing something wrong.
1) Run the page by users unfamiliar with it, and see if they'd install it, why, or why not. Especially language learners (e.g. go into a Chinese classroom). DON'T tell them what it is -- users don't have you to walk you through it ("I wrote this app. I'll buy you a coffee if you tell me what you think of it....")
2) Explain what the app is on the app's page. I can't tell.
3) Definitely show a clear feature table comparing free / paid. Is it usable if I don't pay you?
4) Shoot a video showing the app in use. That's super-concrete.
4.7 stars suggests it's good. 73 ratings suggests no one is using it. Your landing page is horrible. That connects the two.
I'm also not sure about the pricing model. $80/year suggests a pretty complete course. Is that what it is?