Well generally when we get a call IP from a guy like this, a typical reg. We assume he has a suited connector type or small to mid PP.
I'd generally agree with your statement that he'd probably 3bet AQ pre.

If I were to put him on a hand the way this played out, I'd say a flopped flush with SC type. The thing is I'm not sure how much people just call with the flopped flushes. I know I tend to fast play them in case more spades come and ruin the action. But it's definitely possible. The turn he wouldn't like too much and just call. And river would be easy value when you check.

He doesn't look like someone that would flat QJo with Js.

I really just think it's a slowplayed flush as I don't see a double float on this board. I just am not sure I like the way your opponent played it.

River looks like a fold to me, we hold the Ks so block some of their flush missed FDs. I might have slowed down on the turn and went into c-c mode, not sure how much value there is by betting with the Ks in our hand.

Flop: I like betting here but I probably go a little larger for value. Closer to 75% of pot seems about right to me, we have a strong hand with outs to the nuts and want to maximise our value here.

Turn: When we get called and the Q hits I probably check call here. Opponent can easily have some Qx here, especially with the x of spades and when we double barrel here I think we will fold out a decent number of dominated Ax as well which is an awful result for us.

River: This seems like a reluctant fold to me. It's hard to see what villain can be taking this line with that we beat, especially when we block the K high flush draw.

I much prefer check calling the turn here. Villain will have quite a few Qx combos in his range making a value bet pretty thin. I would intend to check fold the river if villain bets twice.

As played I'm definitely check folding the river. The only hands he's ever bluffing with here are a random bare missed spade draw, which this player type shouldnot have much of.