Villain is more competent than your average 2NL player.
C9 giving a 2NL player credit? Not sure if I like this.
I seem to get a little confused in similar spots to this, I'm not usually 500bbs deep but.
Villains range after calling flop raise could consist of any spades, any decent overs, any connectors within that flop.
66-44,ATs+,As9s,As8s,Ac7c,Ad7d,Ah7h,As6s,As5s,As4s,As2s,KTs+,Ks9s,Ks8s,Kc7c,Kd7d,Kh7h,Ks6s,Ks5s,Ks4s,Ks2s,QTs+,JTs,Ts9s,9s8s,87s,76s,65s,54s,43s,32s,ATo+,A7o,KTo+,K7o,QTo+,JTo
River
Rather than looking straight into the turn I would decide what I would be happy with when the river hits.
4 (non spade) - We have the nuts. There are not many of villains
hands that this will improve to get stacks in though. 34 or 44 we might get a decent amount from him - Build the pot
9 (non spade) - We have second nuts but I don't think he would have gotten this far with T8, maybe T8 of spades depending on how loose he is and your aggressive image. Wont do much for villains hand but if he has 2 pair or a set he is less likely to put us on the straight. - Build the pot
8 (non spade) - We may well be good here over any A7, K7 hand but no good against any connectors that could have gotten here (67, 78) - We want a small pot here.
Any brick - Our hand will never hold up. If it is a scary card (another 7 or maybe even Q,J or T) and we decide we have some fold
equity we would rather have a smaller pot to
bluff into rather than a pot that is suddenly 150bbs. - Small pot
Any spade - How often do we see him with a higher spade draw here? I think it will make up about 30 or 40% of his range. How much fold equity do we have on this turn against his spade draws? Will a 2/3 - 3/4 pot size bet get him off them? If so we should be betting to fold them out. If not we should be checking and calling a river bet if one hits. - 50/50
So there are 8 straight cards in the deck that we want to build the pot for, 29 bricks that we want a small pot for and 9 spades that depends on the player and our image etc. I think its only the four 9s in the deck that we could get stacks in with so the implied
odds are not great.
Turn
You have said that you have raised his flop bets a lot, I guess he has been folding to your raises a lot.
I don't really think he will fold much on this turn that he called your flop raise with. Any A7, K7, 78, 88-99 even some middle pairs like A3, 34. Obviously any 45, 67 won't be going anywhere.
Whats our plan if he check raises? The pot will be around 300bbs but I think we will still have the odds to draw as he will be doing this most of the time with a made straight or a set and we could discard all of those Ax, Kx spades hands that we had him on on the river and our implied odds will be huge.
End
In the end I think checking behind here is the best option as I don't see him folding to a turn bet all that often, there are only 4 cards in the deck that give us good implied odds on the river and even if we hit our flush we really don't want to be playing around a 300bb pot with an 8 high flush.
This is all from a full ring Nit BTW...