Have you considered scaling back the free version to a portion of Hiragana and Katakana? (Maybe half?) And maybe throw in a few simple kanji? My thinking is that there is going to be a large portion of users who can immediately see the value of the app when they first start learning Japanese, but won't ever make it to the point in their own study of the language where they're trying to write Kanji. I think a lot of people don't stick with it.
I've spent a small fortune in my pursuit of Japanese (and would probably buy this too, if it were on iOS.) But if I only paid for the books and tools I stuck with or got more than cursory value from, I would probably reduce my purchasing by about 90%. My wife just said 95%. :)