I highly recommend focusing on this before getting to sentence mining. Unless you’re really good at remembering kanji readings and meanings, you could be setting yourself up to forget them before you get to the words that use them.
Then again, if you’re currently actively reading, you may be seeing the kanji in the material you read, so that may not be an issue for you. I guess it depends on how much you feel you’re getting out of doing vocabulary through WaniKani, versus using the site only for kanji.
Personally, I do all my vocabulary lessons before I get to any kanji lessons, but I also don’t mind the slower pace that this results in.
The hardest part for me on starting sentence mining was that it was hard to find sentences with just one thing I didn’t know.
At first, it seems simple: “This sentence has only one word I don’t know. Making a card for a sentence with one unknown thing is considered the best way to learn.” (And indeed I’ve failed at mining attempts with multiple unknown words.)
But then I realize, “This unknown word has two kanji I don’t know. That means there are three unknowns. Unless I count the meaning and reading for the word separate, in which case it’s four unknowns.”
Sure enough, I fail at “one unknown word” sentence if I don’t already know the kanji.
Initially, I started sentence mining only if the unknown word included kanji I already knew, but I was weak at remembering the reading, or if I couldn’t figure out the meaning.
More recently, I’ve been using the Migaku Kanji God add-on for Anki, which auto-generates kanji cards for me. This allows me to create a sentence card that has an unknown word also with unknown kanji. Depending on volume of cards and number of lessons done per day, it’s possible to do the kanji card lessons a few days before I get to the sentence card lesson.
This helps reduce a sentence card from “unknown word reading, meaning, and kanji” to “unknown word reading and meaning”, which is a bit more manageable.
I don’t do enough sentence mining to really say if this is helping me out, though. I’m currently still using WaniKani as my primary source for learning new kanji.