[Kalledrengen]

Nah you are not imo, you have the nut straight. It's way more likely he has a FD rather than a made flush. But I guess it also very much depends on reads. How often he reraises your c-bets ect.

Regarding extracting max value, it's hard giving input without any reads on villian. With some villians you easier extract chips from them by giving them the oppotunity to bluff. Meaning a call here would probably extract the most value because they would shove the turn in a last desperate try of taking down the pot.
However it's a scary board and facing a shove when a diamond hit the turn would be really ugly. So I like your shove and would probably have done the same with that board.