I'll reraise, but only raise to like 700-900ish, if he shoves i'll fold
You don't have position, so you want to be the aggressor
Calling should be fine, but there is not much implied odds since he'll be aware of the flush too.

I'll check here, if he bets I'll instant shove.
I don't mind if I get a free card, since my pair of Ace has some showdown value, and I'm on a flush draw as well.

I put him on a pair of Jacks with a decent kicker, like QJ, KJ
Or maybe lower 2 pair or sets, Q high flush draw
premium hands very unlikely, since no ones want to limp in with AA against 6 players, unless he is a donk
Also is your opponent a calling station? Or capable of folding decent hands?
Fold equity is also a factor that needs to be considered with

Lol too many editing.
Good stuff our thinking pattern is similar.

betting 500 is a bit small and suspicious, if he just simply call then it is very unlikely he has 2 pairs
Slowplaying sets also not too possible, because the board is too dangerous
I think he has a pair, looking for a cheap showdown or on a draw. As the bet is too small, any drawing hand will call here.

It will be funny if he actually had KQd and a diamond 9 came which gives you nut flush and him a straight flush

So now you have the nuts, what you need to think is to extract most money of that guy.

I'll bet around $1000, pretend to buy the pot cheaply
This bet is very reasonable, a Q high made flush will reraise, which is what you want to see
the pot is around $3000-4000, even a single pair should be willing to call with $1000

I won't do a checkraise here, because your opponent hasn't been showing strength after the flop
and he might check down with hands like baby flush or 2 pair.