You should definitely write kanji, writing katakana or hiragana alone will not make much of anything, since those forms are pretty basic and are kind of unrelated in most cases to actual kanji. For example all of the hiragana that have rounded forms cannot be within kanji because kanji are square shaped.
I've used WaniKani and while it is a great tool, it is not a replacement for writing, you can use it to learn words, but you still need to write, because otherwise the way your brain saves the information in your memory is not the same, you learn to recognise kanji by their general form rather than by its composition. At some point, and I can tell you from experience, it gets hard to differentiate between very similar kanji unless you write them and know exactly which part goes where.
If you don't want to write anything at all and just go by using the keyboard, which I don't recommend because of what I've said above, you're better off not writing anything at all, writing hiragana/katakana won't do much of anything.
I recommend you to go with the Heisig method, although maybe it's not for everyone, but in my opinion it works.