kitsun.io is the way to go imo. It’s a product being built by the level 60 WK user @neicul, so using it feels very similar to using Wanikani, and it expands on a lot of features to improve one’s learning.
Something to take into consideration is that even though Kitsun is free for now, it will become a paid service on the 1st of December (with 14 days of trial).
You can see lots of screenshots, features and prices on Kitsun’s thread: Kitsun.io - Web & Apps SRS Study Platform
In terms of method of learning words, there’s several different decks available that could attract you (login to access the links):
- Genki Complete Vocabulary - Genki is probably the most mainstream textbook to start with grammar. This deck has all the vocab you’ll find on the textbooks, ordered by chapters. By knowing the vocab used there, you can focus better on learning the grammar.
- 10k - Kitsun Optimized - This deck has the 10000 most common words in Japanese. I’m almost finishing it, and I can see that it greatly improved my vocab.
- Katakana 4.5k - a deck with 4500 katakana words, ordered by frequency. It seems very easy to understand/read katakana words without learning/using SRS on them, but if you don’t learn them, you can’t use them in your active vocab. Japanese vocabulary is not just kanji words after all. I’ve finished this deck and I love how easy it feels to my brain to recall katakana words and use it in speech/text.
- Japanese Verb Conjugations - A deck to help you practice verb conjugation.
- Beyond WK Kanji - A deck with around 1000 kanji that WK doesn’t teach (mind you, WK doesn’t teach them because they’re quite rarer).
As an example, this is how the Genki deck looks like (made by our lovely @hinekidori):
You can also create flashcards of words that you want to learn in a very simple way using Kitsun, as Jisho is integrated on the website. You just need to search and in a few clicks the cards gets created 
I’ve been following the development of this website from the very beginning (May 2018) and I can say that the creator is very hard-working and dedicated to building a good product. If the fact that it will be pay to use is something that doesn’t attract you, I’d recommend Anki.