If you’re looking for simple books, I would recommend Natively. The account is free, and you can sort books by free, if money is a concern.
You can also sort the books by level. I’ve been slowly working through the low-level ones in my spare time, and I feel like a young kid reading their first books in their own. Which for Japanese, is exactly what I’m doing! They also start off a lot easier than NHK Web News Easy. (NHK says it’s Easy, but it’s still a jump at first).
Between Natively for books, NHK Easy for news, and Jisho.org for the many words I’ve not learned yet, I can feel like an adult learning a language, but still accept that I’m at a kid level in terms of understanding.
I’ve been studying for about 6 months and finished N5 grammar, and I’m really hitting N5 vocab before moving on. I’d started with 3 grammar points a day, but that turned out to be a bit much. I felt like I was getting in too deep, because I’d not fully mastered earlier things before I was trying to build on them. I’d felt sad and hopeless and cut it down to 2 grammar points per day, hit the same wall, and cut down to 1 a day.
Something that helped was resetting progress on some of the grammar points. Around lesson 8 I’d gone through the Cram function and found all the points I didn’t just know, and I completely reset those points. And all my points with ghosts. And all my Troubled Grammar. Learn them from scratch, like I’d never seen them before. But I had seen them before! Like a week ago! It wasn’t totally new this next time, and seeing them again wasn’t so hard.
If grammar isn’t sticking, it’s okay to start fresh. Learning a language is new and exciting, especially when it’s so different from your native language. But it can also be overwhelming. Just take your time, and if today was rough, then tomorrow is a new day.