You can use the genki books right from the beginning. It really starts at zero (case in point, the first two chapters still give you romaji readings in case you don’t know your kana yet). They start of quite easy, though they aren’t super well suited for self study (lots of exercises with partners and the like), I’d still advise you to get it as soon as possible (or some other grammar resource, Tae Kim’s is amazing and best of all, completely free).
If you like to learn Japanese through Japanese and not English, you may like Jalup. You can study with an Anki deck or an App. Basically they start with one word and build up from there, one word at the time. The first 100 cards are free so you can test it and see if you like it.
I used Genki before (in self-study) even starting with Wanikani, learnt some kanji from there beforehand but it didn’t really stick. It went further to university where we used a book that was written of one of the professors there and well, we switched afterwards to Marugoto (the A2-B1 book) which showed so many kanji at once I didn’t know so I felt left behind.
So that brought me to Wanikani.
Now, I picked up Genki 2 again for grammar revision. And Genki is a book for which you don’t need kanji knowledge (as it has Furigana) so you can start right away with it. I would wish though they would leave the Furigana out but they only do in the kanji section at the end of the book. But I think about working through that once I finished the other parts of the book.