Anki, even though it takes some getting used to. Some Supplemental Material (Anki Decks)
Personally I changed the card layout of those decks a lot, but they’re a good basis.
If not Anki, definitely FloFlo (FloFlo is more user friendly imo)
I’m not too big a fan of “Core 6k” or whatever decks, which basically teach you the most common vocabulary. Instead, I’d advice you to study things like the Yotsubato! Reading Pack or pick an Aoitori Bunko book on FloFlo.
It doesn’t matter that you can’t read those books yet. Pick a book, start studying the vocab and once you’re far enough into grammar you’ll already know the vocab you’ll need for reading that book. Way better than just learning common words. (Most vocab in a slice of life childrens book is pretty common too, so it’s not like it’s less useful. Makes for a good study goal too: Read book X)
With Floflo you can even set your WaniKani level to something higher than you are right now (e.g. 20) and it will exclude all the vocab that is also on WaniKani. Then you can read that book once youre level 20.