I created a website for checking Japanese text difficulty: https://kanji-stats.tomaszoledzki.pl
You can paste a text of your choice (song lyrics, news article, wikipedia article) and you will get a bunch of interesting numbers and charts in return. It uses kanji frequency data so you may see what part of kanji are common ones. You may also provide your WaniKani API v2 key to get additional information (e.g. what part of kanji in the text you already know according to WaniKani).
Feedback and ideas for improvement are very much welcomed.