When should I be looking at multiple learning resources?

Short version: don’t be like me. Start your grammar learning now. xD

In order to actually understand anything, you will need grammar. I haven’t done the DuoLingo course, but I have seen discussions on here saying that its grammar explanations are a bit lacking, so you will probably want to pick up another source. Many people will recommend Genki I or Tae Kim’s guide, but there are a lot of other options (check the link below). Genki definitely has the most exercises, but it can be expensive (Tae Kim is free, but doesn’t really have exercises to practice). I have been majorly slacking on my grammar studies and I am regretting it, so I definitely recommend you start as soon as possible. ^^

I think you could probably hold off on Anki unless you want to use it to reinforce vocab from whatever grammar resource you use and/or learn kana-only vocab (since these words are not a part of WaniKani). I just started doing a Memrise course with kana-only vocab, but I probably could have and should have started earlier. Personally, I don’t really like Anki, but I think I’m probably in the minority. ^^ There are a lot of Anki decks/users running around on the forum - I know a popular Anki deck is the Core 10k deck and derivatives (check the link below). If you do pick up Genki, I believe there are pre-made decks with the vocab from that as well, so you probably wouldn’t have to build your own deck unless you really wanted to.