You seem to take for granted that above levels 15-20 you can learn Kanji in an easier/more pleasant way to you than using Wanikani. If this were the case, of course it would be wise to do it, every person learns in their own way.
I can only tell you about my experience: I started reading with the Wanikani beginner book club when I was about level 9 and I am reading my second book now. First of all I have to say that the people participating in the book clubs are immensely helpful and thanks to them I can read and put some of the kanji and words I have learned with Wanikani into practice.
I have noticed that when I encounter a kanji I haven’t learned yet it is much more difficult for me to remember the word than when I already know the kanji but not the word. What I want to say is that I’m finding the knowledge I’ve acquired with Wanikani very useful for reading. And through reading I expect to learn much more.
Edit: Just corrected some typos.