I'm at level 2 right now, what resources should I study alongside WK to build my foundation?

As I see it, The four main points for a well-rounded mastery of the language are Grammar, Vocabulary, Kanji, and Listening/Speaking. There are a lot of great resources out there, so I can’t say exactly what’s best, but unless you want to focus specifically on improving in one area, you should put together a study plan that hits all of these.

There are many good textbooks for learning grammar, but the most commonly recommended (and my favorite) is the Genki series. It consists of two textbooks and two workbooks, and does an excellent job of teaching grammar and building up some initial vocabulary. It teaches some kanji too, but you’ll find that WaniKani quickly outstrips it. https://bunpro.jp/ is an online resource as well.

For vocabulary, Anki seems to be the popular choice, though it’s just a flashcard program. You’ll need to find or make cards to add into it. Personally, I think Genki and WaniKani provide enough vocabulary while you’re doing both, but there are popular decks with a few thousand words available for quickly building up a strong base of vocabulary.

For Kanji… You’re here. Obviously, I’m not gonna recommend anything over WaniKani, unless you’ve got 3 or more hours per day to dedicate to Kanji study. In that case, you may do better with some custom Anki decks, or Heisig’s Remember the Kanji books.

And for speaking/listening, you’ll eventually want to use immersive learning and live conversation, but that’s simply not useful until you’ve built up a strong base of grammar and vocabulary. My #1 recommendation if you can manage would be to take classes. Language schools are prohibitively expensive, but depending on your situation, you may be able to take classes at a local college. If that’s not an option, there are programs and audiotapes designed to have you listen, comprehend, and respond in Japanese. My personal recommendation is Ultimate Pimsleur’s Japanese, an excellent series of language-learning tapes. Many libraries have these and lend them out for free. Just find a place you can talk freely, listen, and do your best to respond aloud. You’ll find it helps your speaking quite a bit.

All of the references I’ve mentioned should be listed in The Ultimate Additional Japanese Resources List, along with others I haven’t mentioned. Go ahead and give it a look.

Don’t be afraid to try a resource and give it up in favor of another one. Find what works for you and stick with it. The best language learning method is the one that gets you to stick with learning the language.