Wait to subscribe until after you’ve finished all their free levels (even though it’s only $5 USD a month, its a waste of money otherwise!). I think they introduce new words in random order. For learning hiragana and katakana it might be faster to just pick up a manga and look up every single hiragana/katakana you find until you know them all.

Readthekanji gives you kanji and vocabulary. All advanced Japanese grammar is actually just vocabulary words, so it does give you grammar too in a sense. What I mean is, past verbforms and a few “particles” literally everything else that’s ever called “grammar” is just vocabulary words.

The book series I mentioned (A Dictionary of…) gives you grammar. The website page (“quick and dirty guide”) is a basic summary of Japanese grammar that will get you to learning faster than the book series will alone. But all the detailed grammar usage, related ways to say things and when to choose which one, example sentences and so on are in the grammar book series. If you seriously want to learn Japanese you will have to use the book series but I think “starting” is easiest with the webpage.

The website (Botsan) gives you easy reading practice. It’s not “lessons”, it’s just something you can start reading right away in order to practice hiragana and katakana or basic grammar. Just because if you pick up something else as your first reading practice you might be a bit discouraged because it’s too difficult.

It’s NOT good to only pick one source and stay with it - the reason is that most sources are completely horrible at teaching something-or-other. Are you going to struggle along for weeks trying to understand something, when simply reading a second book that explains it in an entirely different way would clear up your problem in an instant? For example, a lot of people struggle with Japanese sentence structure but it’s extremely simple (there’s literally only 1 rule you have to follow). Most books don’t even teach it however. Or, most books don’t teach that verbforms are just compound words, so they expect you to memorize something long like “samukunakatta” (“it wasn’t cold”) when in reality that’s a compound of 3-4 separate words. If instead your book teaches you the pieces and then shows you how they’re put together, you’ll understand and remember a lot faster.

If you stick with only one source, you won’t learn those things that help speed up your learning the most. But only switch sources when you don’t feel like you understand what you just read, because there’s no need to read different versions of stuff that’s easy to understand, it just wastes time.