He doesn't need the nut flush to call you here. In fact, with your line i think you get called by alot more than a flush. Lets say you held AK or AQ of spades, would you flat call his flop lead? And would you raise his turn bet? Personally i don't like you line as everything about it screams bluff. Of course the board is so scary that you feel villain needs a strong flush to call, however a smart player that runs through this hand will likely be calling you with top set.
First you say that the villain will call with a lot more than a flush. Then you say that he needs a strong flush to call. Which one is it?
Also, nothing about how he played this hand screams of a bluff. I think it screams of exactly what he's trying to represent here, which are 2 big spades in the hole. If he thinks the villain has a set, why on earth would he raise the flop with 2 big spades and let himself get bet off of his draw? He wouldn't.
Personally, I like dumping a hand like AQ on a board like that when a rock leads into you. He most likely has a set or a small overpair and you're relying on scare cards to fall to be able to bluff him off of a hand. Basically, you have no hand and are probably drawing dead and you're planning a bluff that can't work on its own, but needs specific cards to fall in order for it to be successful. It seems like an unnecessarily risky play in a pot where you really haven't invested anything. The pot is small, a rock leads into you, just dump it on the flop.
Ok, so that being said, your play perfectly represents the hand that you're trying to represent. You're not flat calling on that dangerous board with an overpair, you'd raise. If you did indeed have a flush draw, you'd probably flat call the flop (in position, so as not to be bet off of the hand) and then make a modest raise on the turn when the flush card hits, all of which you did. One the river comes and does not pair the board, if you had a flush, with the pot as big as it is, you'd almost certainly push.
Based on how things played out, it really seems to me that the villain has a set and he should probably dump it on the river. Your bets have told him that you have a flush. He stuck around on the turn becuase the odds weren't too bad to do so in hopes that he could boat up. When he doesn't, I'd imagine that he's gotta fold to that bet on the river.
So, after calling on the flop, I like the line that you took to win the hand. The cards cooperated to get your mission accomplished, but I really think that the correct play here, pretty much every time, is to just bail out when he leads the flop.