I will disagree with people saying that most chilidren books are written in hiragana (only).
While I haven’t looked into that many series, most of the books I’ve bumped with have kanji, and actually progressively they kind of encourage kids to use the kanjis that are suppose the be known at their level (they will wean off furigana gradually).
I have several books from the 10分で読める series. And yeah, the ones aimed at １年生 have more hiragana than the rest. But they do have kanji (with furigana) and the parsing it’s quite good. Believe me I’ve faced some terrible full hiragana text too (those are nothing alike).
If trouble is content, the 10分で読める伝記 series are biographies. So there’re not plagued with super fauna and magical creatures (if that’s not your cup ot tea ).
Overall mixing this reading with graded readers aimed at japanese learners it’s a great mix. The native content (aimed at kids) will be basic in content but will have more grammar been used, and vocab will be much more than graded readers. Graded readers for japanese learners will have controlled grammar and vocab (much less new words than children books), but usually will try to tell a more complex tale and eventually they become more dense than children books.
As for the rest, conversation it’s nothing I’ve tried too much; I’l be spending my first weeks in Japan after a year or so of studying, so I only hope for the best … anyway, I think listening it’s like the main thing even before getting too chatty, bacausel… it’s a two sided thing … so you might get accustomed to hearing whatever they will say you first. Probably you could notice this by yourself if you start with a tutor, so there’s that.
Anyway about adult or even young-adult readings… I understand that content wise some people will prefer to dive into that… for me the main issue with going that route it’s vocab … even in a children book I bump into 2-3 new words per page (and I’m suppose to know already a bit more than 4K words by now), and those books aren’t too dense in word count. So do the math and you might get the idea of how much your nose will be in th dictionary with a more adult aimed material … Japanese books can be made into a very gradual immersion material, as they are very well tagged regarding audience, that will help to increasingly test your new skills.
I posted some examples of progressively nore difficult text in another post, you may check it to see an overall picture of the difficulty presented in material aimed successively at older audiences.
ps: about the Japanese the Manga Way. It’s a great reading, but I wouldn’t depend on it as a systematic grammar tool. More like a cover to cover reading, to get some nuances that more systematic approaches might ommit. That and Kawajapa yt channel do a similar thing regarding grammar. A good reference gammar book it’s a must if you are not too much into textbook learning