2/4: Top two pair facing a River Raise

€2-4 NL 9-handed
UTG+1 Hero 450€
CO V1 700€
BB V2 900€

I just switched tables a minute ago and this is the first hand I'm playing here. V2 is a regular but V1 is someone I don't remember ever having played with before. He looks late thirties, buff, very relaxed. Was talking throughout the hand up to the River with someone else, didn't seem very invested.

Hero is dealt A♥ K♦

Hero opens 12€, CO and BB flat.

Flop (42€😉 A♣ 3♦ 9♥
V2 checks, Hero bets 30€, V1 calls, V2 calls

I was betting a bit larger than normally as a basic exploit (an Ace will pay anything and there aren't too many draws), and I'm not worried about balancing vs. these players. If it were heads-up I'd have considered checking as well.

Turn (132€😉 4♠
V2 checks, Hero bets 70, V1 calls, V2 folds

This is a reasonably safe card so I just bet again.

River (272€😉 K♥
Hero bets 100, V1 raises to 300, Hero ???

I think this is a straight-forward spot to go for a third value bet, even without the second pair. But I don't think Villain's holdings are super strong (probably just a mediocre Ace) and not a lot of people triple-barrel in this field, and also people tend to think I'm not capable of triple-barreling as a bluff. So I don't think going huge makes a lot of sense; I think if I bet 200 or something, he'll only pay if he has two pair. So I just bet 100, hoping to get a begrudging call from a weaker Ace.

Then he raises, and this raise is just really gross. The problem here is that the Flop was so dry that a slow-played set makes a lot of sense. I definitely think I'm vs. a set here most of the time. If there was no money in the middle, I'd fold for sure. But is it a set 3/4 (77%) of the time? Because that's about the price I'm getting. The only other hand that beats me is 52, which doesn't seem super likely both because of the Preflop and the Flop action.

I think the probability of this being a pure bluff is probably well below 20%, but could he be value-betting two-pair? Is this A4? A3? A9? Maybe also AK (it's actually not that unusual for people to just call with AK)? Maybe someone value-betting a random K9s? That seems like a stretch.

Idk, I was really torn here between "raises on the River are famously value-heavy, this is always a set" and "you're trying to be too cute here folding top two pair when you get this good of a price, you have to call". I also wasn't sure how if at all to weigh him talking so freely during the hand. (Would a set be comfortable because it's strong, or would it be excited for the opportunity?) Fwiw he also didn't think too long before calling Turn and Flop (though it wasn't an instant-call, like a few seconds for each). Definitely there are a lot of people in this field who would have raised a set earlier. (But then again he's probably not a super strong player and raises on the River are so value heavy... )

(Edit: messed up the transcript at first, sorry, but fixed it about 5 min after posting.)