That's very cool. Thanks. So you solve every RQ in an era until you've gotten its diamond reward, eh?
That is what I did. It is also one of the reasons I prioritized CF. It will increase the output of diamonds. Even level 10 brings it from 20 to 50. Currently, would get 109. In the old days (and maybe again), some of the events didn't have items to collect to spend (like the current cherry blossoms), you would just get random rewards and one would be diamonds.
To answer the original question. I spend them on:
-Expansions (best use of them in the game)
-GE if I know there is only one answer left and I want to move on
-GE if I make a stupid mistake (same wrong item 2x in a row), I pay to fix my mistake as a punishment to myself
-Cultural Settlement: I've spend diamonds twice when I'd deleted a ton of stuff to place runes and realized that I was missing a small amount of the goods I needed to open everything I planned. I paid the diamonds (15 one time and 25 the second time). It wasn't worth the multiple hours to just freely fix the mistake so I paid for it. Since i get all my diamonds from GE/events/RQ, I didn't mind.