WaniKani covers a lot of vocabulary. However, it covers it from most simple kanji to most complex kanji. For example, 大変 is a very common word, and anyone who’s watched even a bit of anime will likely have heard it. However, while 大 is a level 1 kanji here on WaniKani, 変 (rightfully) doesn’t show up until level 15, at which point the vocabulary word appears.
I’m a fan of using iKnow! for vocabulary. However, this is partly because I had an unfortunate bad experience with the Japanese Core 2000 deck with the SRS flash card program Anki. (The deck wasn’t sorted, so I was thrown right into the middle. This was my first experience using Anki, and had no idea about sorting the deck.) Since iKnow is a subscription application (like WaniKani), I might choose Anki (free, offline) if I had it to do over, but I have a lifetime subscription with iKnow!, so no reason to switch now. I’m at almost 2,000 vocabulary cards on iKnow!, although I almost completely stopped learning new words with it last year when I started reading manga and learning words that way.
Regardless of whether one uses iKnow! or the Japanese Core 2000 deck (via Anki or another SRS flash card service or application), you can’t go wrong learning vocabulary starting with the most common words as these decks cover.
I also started using Bunpro (another subscription flash card site like WaniKani) for grammar. However, the more time I started putting into WaniKani and reading, the less time I’ve been putting into thoroughly learning new grammar points, and trying to understand the areas I’m struggling. I need to see if Bunpro has a feature similar to a level reset in WaniKani, then slow down with it. (Not that I was going that fast to begin with. And, yeah, there’s no way I’m finishing N4 grammar this year.)
Last year was my first year reading through a whole volume of a manga in Japanese. My number one resource was Google. I’d type in the portion of a sentence I didn’t know, and add “meaning” or “grammar” to the search, and would often have at least three really good resources come up.
I also like ichi.moe because I can type in a sentence in Japanese, and it’ll break it down into words and conjugations for me. Sometimes I find phrases and expressions I wouldn’t have discovered if I tried taking the sentence apart one word at a time on my own.
Remember, the faster you learn new kanji, vocabulary, grammar, the better chance you have of not absorbing it all. Make sure you’re understanding the material properly before moving ahead. (In the world of WaniKani, this means learning the mnemonics, as they’re the tool that makes it easy to learn new kanji and vocabulary.) Take your time, figure out what pace works best, and if you feel you’re getting overloaded, there’s no harm in focusing on reviews and holding off on new lessons.