Originally Posted by Ubercroz
By flatting the turn I disguise my hand strength
If you think this is the case (and I agree it is), then why are you even considering folding river?
By flatting turn you induce worse hands to value bet (straights, ALL flushes, not just Q/A-high and straight flushes, once in a blue moon a set or two pair), and also induce bluffs.
You clearly beat his range, have underrepped your hand tremendously, and if he has a flush he's basically never folding to a raise (weak players don't see hands in relative terms, but in absolute terms. IE "I have a flush, I don't fold", not "I have the 4-high flush and I don't expect my opponent to shove worse than maybe the Q-high or K-high flush, so I have to fold even though I'm getting a good price".)
Just shove - people can't fold flushes, and if he happened to play a straight this fast then he's pretty likely to pay you off too.