And still you're fluent in English (I assume) - it's not an issue with the English language but your understanding of the context. Japanese (and Chinese) work a bit differently than latin languages. Even if you know 100% of the kanji (very few Japanese people do), it doesn't mean you know 100% of the words - and vice versa. Since the characters are idiomatic, it also makes it easier to guess the meaning of a word or character you don't know than if they would have been written out phonetically.
Kanji: The Chinese characters in question
N2: The second highest level of the Japanese Language Proficiency Test (JLPT). N1 is by many seen as a very high bar and many Japanese employers demand it if you'll be using Japanese as your main working language. N2 is generally attained after 1-2 years of Japanese studies.
> Since the characters are idiomatic, it also makes it easier to guess the meaning of a word or character you don't know than if they would have been written out phonetically.
Ideographic, as in trying to portray ideas graphically, not idiomatic, as in using expressions like a native speaker
> "N2 is generally attained after 1-2 years of Japanese studies"
From my own experience, this would only be true in the most favorable of the situations. 2 years studying fulltime while living in Japan sounds about right. 1 year studying as a hobby few hours a week, no way.
Do kanji handwriting-recognition models even “know” (i.e. are trained on) all in-use kanji, or do they just not bother with some of the rarest ones because their presence in the dataset would decrease the likelihood of correctly matching more-common characters, which it’s overwhelmingly-likely are what you’re writing?
Presumably it's the same as words in English, where you'd never make a word suggestion model that just blindly considers all possible words. You swipe jow or gow and it will change it to "how" despite the other two existing in arguably correct English in some very obscure case.
Kanji: The Chinese characters in question
N2: The second highest level of the Japanese Language Proficiency Test (JLPT). N1 is by many seen as a very high bar and many Japanese employers demand it if you'll be using Japanese as your main working language. N2 is generally attained after 1-2 years of Japanese studies.