Try this for your original thing:
var raw_data = UrlFetchApp.fetch("https://www.wanikani.com/api/user/d8fd194bf2f2b9f7275b795f62e4b13e/srs-distribution",
Thank you, it is much clearer now!
Based on the API docs, this v2 API query may be useful.
https://api.wanikani.com/v2/assignments?srs_stages=5&subject_types=kanji
Assuming you properly authenticate it with your v2 key, this would return assignments at SRS stage 5 which are kanji.
Do note that the categories Guru, Master, etc. will be removed from WaniKani in the near future.
.






