>>1920772
I am somewhere between N3 and N2, so I don't know much about N1, but I can give you advice as an ESL with a Cambridge C2 Certificate, meaning I am about as good as you can get for a non-native. Going at least once or twice through a grammar book, to at least be familiar with the concepts will definitely help. Now it also depends on what your goal is, if it's simply watching anime, reading manga, and playing games, then you can just look up terms/grammar points whenever there is something you don't quite get, I mean it's what I do right now, so it will work. If your goal is to pass the N1, then I guess you should grind the books, until you can pass those mock tests.
One small caveat about me and the english grammar. A lot of English I learned though immersion, as for whatever reason, the cartoons that were on TV were just in English, so I was doing immersion ever since I was born, plus videogames and much later Youtube, movies always had subtitles so they weren't good for immersion. At school I was very good at vocabulary, listening, essays, but shit at grammar. I was doing things "intuitively", but I was bad at it. Basic stuff like, "after did use verb in present tense, instead of past tense" was something the teachers had to spend years trying to correct me on, because I felt like it had to be the past form, it was what felt "right" to me. My father who thought I was good in English, because I could speak some words after years of watching TV, was horrified when I said stuff like "He go" or "She go", instead of "He goes" and "She goes", so he sent me to private lessons as well. What I am trying to say is that, it's possible to make wrong grammar connections, when just doing immersion. I basically
grinded ground
apparently "to grind" is an irregular verb, and I was unaware of it Grammarway 4 for years, before taking the Cambridge English Proficiency test.