Welcome, glad to have you here!
I’m sure people in these forums will have all sorts of suggestions- as different learning resources work well for some, not as well for others. Some of these might click, some might not- but;
Jisho.org which is an excellent Japanese dictionary to look up any words or Kanji. It’s an excellent resource just to have in the back of your head as you study anywhere, and if you want to practice writing especially it contains stroke order diagrams for every Kanji. I find it helpful for memorization
Anki.org is bound to show up here- it’s free to use though you have to import (or create your own) content. It works on a SRS system like WaniKani but is so flexible you can tailor it to help with grammar, vocabulary, Kanji, or anything really. Many people here use it. I personally don’t enjoy the UI and it can lack some consistency, but it’s a great supplemental tool regardless
Genki Textbooks are often cited as great entry-level grammar tools, as they are very well explained and easy to follow. This gets a pretty great foundation set for everyday conversation and offers lots of intuitive practice. I think they’re pretty excellent from my own experience. If you have the discipline to work from a textbook, can’t recommend enough.
Bunpro.com is probably the most expansive grammar-learning resource out there, with an incredible amount of content to introduce N5-N1 tier grammar. Its database is very thorough, there’s tons of vocab on the site that WK doesn’t cover (especially Kana-only words), and there’s lots of useful opportunities within it. My major criticism is that its SRS system isn’t quite ideal for something as nuanced as grammar, often being a little too vague of what it’s asking and causing frustrating mistakes- but it still is nice for some repetitive practice to help lock in and recognize concepts. I use the site despite my critiques just because it’s very well organized, feature-rich, and not terribly expensive. You can also sync your WK progress on it if you don’t want to re-learn the same vocabulary.
Busuu.org is not without some shortcomings, but I have found it to be a pretty great and informative site all around. There’s great variety in questions- typing, fill in the blank, multiple choice, listening, short stories, etc. Along with infinitely curated vocab and grammar reviews if you just want to stop and practice your content, which I like. It also offers multiple variants on the Japanese courses- so you can do a standard CEFR-scaled course, one focused on the context of travel, one focused purely on Kanji, one that contextualizes it in Manga, etc. And the most interesting part is that you can interact with other learners, so native Japanese speakers can correct some of your exercises and vice versa.
Dictionary of Basic Grammar is not a source I’ve used quite yet, but I see it tossed around pretty unanimously as an excellent tool. Thought I’d mention.
I think once you build up enough Vocab and grammar to be decently comfortable, you can start introducing some larger reading into the lineup. NHK Easy is often praised for being beginner friendly and (being a news site) uses fairly common, everyday words. Yotsuba&! / よつばと！Manga is usually cited as a perfect entry-level manga for those learning Japanese and I can attest to that. Children’s Novels like 魔女の宅急便 / Kiki’s Delivery Service also come up quite often.
If you like WaniKani, I would personally recommend just sticking with it for a fair while before jumping into anything else, at least in a long-term or intensive manner. There is a desire to learn from all corners possible, but it can quickly become overwhelming if you bite off too much at once- and if you take the time to establish a fair lineup of vocabulary, it means you can focus better on the grammar mechanics when trying that side of study or applied reading. Learning Japanese takes a lot of discipline and patience, but also experimentation to find out what exactly your needs are, your available time and motivation investment, desired outcomes, and resources that click. I hope that any suggestions here can help make that process easier, and you enjoy the journey just like I and the rest of us here have/are.
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