Honestly, you can start Genki at any time. The Genki books focus on grammar and a little vocab, while WK focuses on kanji and (usually different) vocab. Neither one is needed to understand the other, but you may find they help reinforce each other.
For me, I really like studying them side-by-side. The grammar from Genki helps me understand how to use the words from WK, and the kanji mnemonics from WK help me break down the Genki vocab and remember them better.
Just do it when you feel like it—it can either be a good boost before you start diving into WK, a good companion, or something to brush up on your knowledge.
Personally, I think it’s a good idea to start with a textbook right away (simultaneously with WaniKani). It takes a while for WK to ramp up, so it’s a great opportunity to make some progress in Genki.
Yes, do both at the same time
WK teaches you kanjis, not Japanese.
You can’t learn Japanese without learning about grammar so don’t put it off. Just do slowly a bit of each to not get overwhelmed.
Also you don’t need kanji to do Genki, there are none on the first two chapters, and then they are introduced slowly and always with furigana. So doing WK on the side is a bonus but not a requirement.