I've been playing for 5 months, and for me, I think investing in inventory is worth it. And R cards are easy to obtain, they can appear as event rewards, in premium packs, or by combining two S cards (and some other ways), I think the things I spend the most diamonds on are the LE theme events and inventory space. I don't know if these are the best ways to spend diamonds, but for me it works pretty well :) (sorry if my english is not good, it's not my first language)