You're welcome for the comment!
I've managed to get the diamond rating on 14th and 18th level. Both of these were shockers for me honestly. I got the diamond rating on 14th level, in my first try, and on the 18th, after like ~50 failed tries (seriously, that one had me thinking).
As to persistent data in my game, I simply used PlayerPrefs to save the high scores, save files, and settings. They also work for the WebGL unity games, although updating these games, or removing cookies, might reset the saved files.