4-card straight, possible flush, small river bet - fold OP?

You bet 6,5bb pre flop, why? If you know that they are fishes and they will call anything pre flop then I understand. I might have had a little bit different line on the turn and river but since we played this way I would fold river because the board is so wet and I assume he is a fish cause he called your megasupermonster pfr so that board hits his range pretty hard.

I think it boils down to: how often would villain just straight up bluff this river?

I agree that after the check behind on the turn he is very unlikely to show up with a decent hand BUT does he turn 1pr hands into bluffs or go for some thin value with a worse hand? On such a scary board after you've already shown strength preflop and on the flop I would doubt it. As a default I would just expect him to check behind all 1pr hands and only bet with 2pr+ here.

i think hand is ok, folding the river is probably somewhat better but given the turn action and halfpot bet it's whatever i guess. just take a note