I think folding the river is fine. I also think that a more interesting question is what to do on the flop. Do I understand correctly that you figure someone had to have the flush draw on the flop so you would fold to a good sized bet on the turn?

I understand with 5 opponents on the flop you fear the flush draw but mathematically speaking it's still far more likely that the flush draw is NOT out there (correct me if I'm wrong) which indicates pushing the turn. Sure you'll be be facing a flush sometimes but I believe you will also be ahead much of the time when called and you will take it down uncontested lots too which is fine also. There's over 600 in the pot and you have about 800 chips left at that moment, the risk reward looks real good to me here. If the pot were significantly smaller or your stack bigger then it would be a much tougher spot but the way things are pushing looks clear to me. One thing for sure you do not want to give a free card here because if you still have chips when the river rolls around you could be in a very tough spot.

I also disagree that the pot is too big to bet, just the opposite, it's too big not to push.