Me too! Japanese is a hard language for me to learn, but Wanikani makes kanji fun.
Joyo is the order that Japanese schoolchildren learn. JLPT, as you know, is for foreign adults. Wanikani is different from both of them, but not too different.
WK level 16 = JLPT N5 100% (N4 96%, N3 51%, N2 25%, N1 3%)
WK level 27 = JLPT N4 100% (N3 83%, N2 53%, N1 12%)
WK level 40 = JLPT N3 98% (N2 83%, N1 32%)
WK level 51 = JLPT N2 100% (N1 55%)
(source)
If you have the time, it’s fine to learn “extra” kanji through WK. Then when you get to it in Kumon it will be easier. Similarly, if you learn a kanji in Kumon first, it should be easier when you get to it in WK.
I’m not really either 
This is the kind of stuff I’m reading for practice:
(It’s a book for 7-8 year old kids.)
This article might give you some more encouragement. It talks a bit about why making mistakes in language learning is ok - and even a good thing!
As Leebo said, I think you’re going to be fine 
Because you are taking what you’re learning and trying to apply it in real life, you’ll remember it better.