You can definitely reach the “elementary” level within 6 months, but WaniKani would not be the biggest part of you getting there. Everything but section IV has furigana on it. So, WaniKani would help you on that one section specifically, and maybe in a general sense for some other vocab. But if you focused solely on WaniKani for 6 months, you’d fail the elementary level test.
EDIT: I checked out the advanced test’s kanji section and it’s decently challenging, I’d say. They ask you to write some high level kanji, which is a little unusual for these kinds of Japanese tests. Usually it’s just reading kanji with multiple choice for kanji-specific questions.