Remembering the kanji isn’t really a textbook. That would essentially be an “”“alternative”“” to wanikani, if you want to call it that. I wouldn’t personally recommend it, the way it’s usually used means you won’t be able to actually use the language until way way later, which is a huge motivation killer for most.
You don’t really need a few hundred kanji memorised to start learning grammar or consuming content. Just fyi, with wanikani, if you go at full speed (which usually won’t happen), you would be learning roughly 140-150 kanji a month, so even if that few hundred is only 300 more, that would still be about 2 months of effort given you are maximising your wk speed, during which all you do is memorise random shapes out of context. I would recommend already starting now-ish, as pm215 also said. Only thing I’d wait for is for the general pace of wk to set in, so you know how much time you have to work with.
Immersion will be a must, exactly what type will be determined by what you enjoy the most (books, manga, anime, games, tv shows, dramas, translating poems, reading twitter, really whatever), but you will certainly need some other external source to get at the very least your basic grammar knowledge in place. I don’t think there are a ton of people on the forums, who’d be comfortable with calling themselves fluent, but here most of us tend to start with reading manga. You can try joining one of the book clubs (specifically the absolute beginner book club), once you are comfortable with the most basic grammar points. We tend to say that an N5 level of knowledge (basically getting through genki 1) is about the level that makes joining comfortable enough.
Personally I went with first consuming the base N5 grammar lessons from as many sources as possible (Genki, Cure Dolly, Tae Kim’s Guide, and various other youtube channels) before jumping into reading a ton. The rest just came as I read more and more.