l0ruscene

A sample size of 21 hands isn't sufficient to get a sense of villain's real 3bet %. However, the fact that he has a PFR of 0 over those hands tells me that he's not comfortable raising preflop at all, which makes the 3bet pretty scary. A FPR of 0 also tells me that villain is a weak player. Based on the limited information we have on him I would just fold, although calling is certainly not terrible. It's just that your hand does poorly against what is probably a value heavy 3 betting range. Turning AJs into 4bet bluff might make sense in other situations but versus the general player pool at 5NL it probably isn't advisable.

The turn is an easy call. Setting aside his bluffs you rightly pointed out that he can have AK or AQ. He could even be taking a funny line with KK, QQ, TT (we have already established that he's a weak player). If he turns over a monster it is what it is. We just need to beat one third of the hands he can have to make this call profitable and we're doing A LOT better than that.

Just wanted to point out that playing AQs as an "almost 100% 3bet" versus an UTG open is probably not ideal if you have a calling range at all (which you probably should and do have). Maybe it's mostly a 3bet if you're using a merged 3 betting range but there's no way it's a 3bet 95-100% of the time.

As played I would just call the river. Villain seems weak and might be playing the same hand as you (as a fish he might even be over playing KQs occasionally or just spazzing, who knows?). Also, by folding such a strong holding you make it quite profitable for him to bluff you. You're getting a good price.