Vocab-wise, there’s going to be a lot of vocabulary in Genki (and any other learning resource) that you don’t learn in WK, since WK doesn’t do hiragana-only words (and a ton of other essential words). I know that’s kinda stating the obvious, but I mean to highlight the importance of learning additional vocabulary elsewhere, because reading is going to be very difficult otherwise.
As far as finishing Genki goes, I believe I finished Genki II when I was in my late 20’s on here. If I could go back, I would’ve started grammar sooner.

