A good aggressive regular playing 25/10 raises to 8 UTG.
I pop it up to 21 with He is the only caller.
He has folded when I 3 bet him a few times before. We both approx 200.

Flop (42)
He checks, I bet 27, he calls.

Turn (95)
He check, I check

River (95)
He bet 45, I call.

I realize that I bet a little thin on the flop.
But what line would you take on turn and river?

based on the way the hand played out, I'd put his holdings on 99, 1010, or maybe KK/AA. Then again you didn't reraise the pot which would have been to $24, so you gave him a little discount to where AQs or AJs might have dabbled.

I like the turn check behind for pot control.

My instinct says you have the best hand 80% of the time here.