Is there some showdown history with his donking hands fro, earlier showdowns? If they have all been value hands, or they have all been polarizing board changes but have led to folds, very different pictures of his likely holdings emerge.

I think it's usually a fold with no flop bet, he bet/calls turn and leads river. But you can't give up too often against good opponents or you'll be too exploitable by floats.

should bet the flop

His heading addresses the river play.

Whereas betting would have been great this particular hand (because results), he didn't make the real mistake of checking out of some habit of always checking the nuts OOP (for deception?). He checked with the plan of checkraising an opponent who bets too often. It's reasonable to check the flop with a plan based off of specific opponent tendencies.

I would fold. What do you think he is checking the flop and betting the turn with? How would the 6s improve his hand if not for a flush draw (the low straight draw and wraps are less likely to lead as there are higher straights that can get there too).

My guess is some combo of high cards + flush draw. Bad players will check call something like a set on the flop but never bet. Then when the flop gets checked through they figure nobody probably dosent have a straight, your raise then looks suspicious and if he then picked up a flush draw he is calling 100%. And if he hits the flush he most likely will bet it for value since he dont think you have a flush.

On the river why would an otherwise passive player bet? He has to think on so many levels if he is going to do it as a bluff and make you fold. I see this so often on flushed flops when they flop a set, they will call and donk full pot when the board pairs. They want you to call, the thinking they would have to be capable of to think up floating two streets and hope the board pairs so they can bluff. They wouldnt try it if they could come with this strategy because the board wont pair often enough to make it +EV

Just the fact of a significant gap between VPIP and PFR should tell you he bets for value when he has it and plays it close to the vest when he dosent.

Easy fold but I know its really hard then and there. Will you get bluff sometimes yep but no way close to what is needed to warant a call.

A villain that has previously showed proper aggression and tried to bully players around might try to bluff but someone who opens almost only premium pairs and rundowns just want to get payed.