It’s totally fine to begin studying grammar now if you want to. 
Genki has furigana, so kanji won’t be a barrier when it comes to learning grammar points. And knowing some grammar will make learning some words in WK easier, imo.
Hello! I’m in a similar position I suppose. I took 4 years of Japanese in college and never heard of Wanikani. It wasn’t until I came to Japan as part of the Jet Program last summer that I first heard of it. I didn’t think it would benefit me much since I had studied in college for so long already, but I felt my Kanji game wasn’t nearly as good as I wanted it to be, so I decided to do the free trial in January, and I haven’t looked back. I love it! It’s still a lot of review at this point when it comes to the individual kanji, but it has helped solidify things in my head a lot better, and I’m already learning a lot of new vocabulary. Its definitely helped me differentiate and remember the onyomi and kunyomi too.I’ve been recommending it to a lot of my friends who are trying to learn Japanese. I know not every system works well for every person, but for me, wanikani is great, and I feel like its very useful no matter what your skill level is going into it.
Thanks for your advice! 
I started WaniKani ~1.5 years into my learning journey. I was pretty sceptical at first but I started learning new words very early on which surprised me and encouraged me to keep going.
A few months later and it has definitely been massively useful. It does a great job at teaching how to read kanji while enforcing it with vocabulary. That being said I did find it helpful not coming in completely blind, and having an understanding of grammar and a bit of kanji/vocabulary knowledge probably made the whole thing less daunting.