Playing 1/2/5/10
Me: LAG player in this game, very friendly "private game" playing lots of hands, generally have a not afraid to play pots image but also considered to be an OK reg.

Villian: first time playing with him but known to all the other players. Action player, seen doing ridiculous stuff all night and generally a punter.

History, about 50 mins before in a 3 way pot I'd called his river shove for 600 in a 2k pot with J7 on 37829 and he announces you win only to turn over 68 (not slow rolling just didn't think he could win after the call)

£2k effective stacks

OTTH.
Me in Button with K9hh, CO makes it 35, I make it 80, Villain cold calls from the £5 straddle, CO calls.

Flop: AsJh8s
Checks to me and I continue for 105.
Villain calls, CO folds.

Seems OK with backdoor everything and in position with the betting lead.

Turn: AsJh8s4d
Checks to me and I bet 375, Villain thinks for longer than usual, looks to be folding but ends up eventually calling.

Usually I'm giving up in 80% of these spots but this was the 20% that was deciding to double barrel and potentially bet very big on any clean runout.

River: AsJh8s4d6s
Villain leads out for £320,
Me. ????

I'll give my thoughts on the river after before revealing what I did and the result. Just curious what others would do here?

Think folding river is the play.

Hero seems like he is forcing the action both pre-flop and turn on a pretty unfriendly texture. The clickback sizing pre-flop is also a bit goofy and bigger is generally preferred.

There isn't enough info in the OP to suggest bluff-raise is a good line on the river - even if bet sizing isn't the strongest, its not obviously weak either. Villain line of call, call, call, lead a river when draw completes is fairly strong and sizing is cheap enough you can bluff-catch/call light.

Unless you have a very strong read that he will fold to a shove, I would fold. This can just be a flush.

If you had a high spade, especially Ks (KsQx, etc), I think you could go for it as you have close to a pot sized raise behind. But as it is, he is too likely to have a flush himself.

Preflop sizing is a huge mistake. Go at least 3x in position, like $105, and I would like $120 or so. You're going just over 2x and this is a big mistake. You should be happy with him folding pre. That is the point of the large sizing, don't give him a good price to set mine when you have AA, KK, QQ, but also have K9s so he has to call with some hands.

Flop size is too big for multiway imo. Honestly you have a questionable hand to even bet with, but you can have AA, JJ, AK, AJs, etc. I would prefer a stab with 1/4 pot sizing. This keeps villain's range very wide, and if you get it heads up, you can bomb turn. Favorite play in these spots would be to hope it gets heads up on turn, pick up BDFD on turn, and overbet.

Turn is just spewey. You have K9s no FD, no equity, no chance of winning, not blocking anything relevant. Unless you have some strong history showing that this guy overcalls flops way too much and overfolds to second barrel way too much, don't do this. You have much better hands to bluff. Spades. KsJx to bluff spade rivers. KJ no spades to bluff non-spade rivers. Straight draws.

You should have lots of bluffs on flops (not really multiway though), fewer bluffs on turns, and the least bluffs on rivers. This means you need to give up a lot of turns and rivers where you don't pick up equity and where you don't hold relevant blockers. Your hand on this turn is the perfect example of a turn give up. And if he checks river, this might be a bluff on river, but otherwise a fold when he bets.

If you make this kind of play, there is a good chance your overall bluffing frequency is just way too high, and you're going to get looked up a ton.

Don't get me wrong, there are spots vs specific players where I overbluff because I expect bluffing with any two cards is likely going to be profitable, but I have to have a very specific reasoning. IE, I am the 3 bettor, I am playing vs a player that tends to 4bet AK who open raised and called my 3bet, flop has A and K on it, I bet 1/3 pot on flop and 3/4 pot on the turn with 0 equity suited connectors, villain folds an A face up. But villain has a huge history of calling flop bets and folding to double barrels, he has very little top 2 pair, no top set, no middle set, likely no bottom set when he doesn't check raise flop. Villain is a predictable regular who has leaks I have identified that I can exploit.

Vs unknown recreational players, you are rolling the dice a lot more. You don't know how wide villain will cold call you pre. Can he have AK? Flushes? JJ? AJs? AJo? Does he donk when his draws get there? Does he have donk bluffs on river? Does he overcall flops and overfold turns? If you don't know, then turn is a big punt. Barrel with the combos that will have equity and block strong hands like 2pair, not just any two.