Not directly connected to your app, but I saw this line:
and wanted to share my opinion, specifically on 銀河鉄道の夜, which I am reading now.
It’s freaking HARD! Even for an intermediate learner.
The book was written in 1927 so it contains lots of outdated kanji/words/expressions/speech patterns (also lots of tree names, flower names, gem names, bird names, strange color names, which is not a bad thing in itself, but can be quite distracting). It is also very abstract and even surreal in the part where they travel on the railroad, so it’s very difficult to fill the gaps in understanding with common sense.
It’s a beautiful book, but not for beginners.