3/5 6-handed (normally 7, but 1 person sitting out), ~600 effective this hand (villain's stack), which took place about 4 hours into session with same set of people at table.

Hero's image at this point probably laggy or maybe very laggy. I'd been getting quite out of line to exploit fish to my immediate right. Fish was open raising like 40-50% of hands and limping maybe 20% of the rest. So I'd been raising & 3-betting the fish probably about 30% of the times he was first in the pot when I had the proper hands to isolate & outplay him postflop.

Main villain is to my immediate left and is almost OMC type. But he had started calling my raises more frequently after about an hour of me exploiting the fish. But he still played quit straightforward post-flop. But I said almost OMC, but not quite, because I did catch him bluffing river several orbits earlier on Q3454 rainbow board, where action had gone b/c, b/c, I checked river and villain potted it for $100 with A8dd. There was no diamond draws.

Anyway, on to the hand. 6-handed, ~$600 effective.
Hero opened $20 from HJ with 53cc. Villain called, BB called.

Flop ($60): 9s4h2d
BB checked, Hero $25, Villain called, BB folded.

Turn ($110): Td
Hero $60, Villain called. (Looking back, sizing was way too small here. Should've been 75-90, probably).

River ($230): 8d
Hero $200 (I think this is about what I would bet if I'd backdoored the flush).

When the flopped came out I already knew I was triple barreling this hand, especially if it got heads-up. Reviewing the hand, wasn't sure if this was good bluff since this board favors the villain's calling range much more than my HJ opening range. I do have all Ace high flush and most of the King high flush in my range, but other than that I guess river bet is repping overpairs which I probably don't go that size. Maybe thinking to triple barrel regardless of the runout was also a mistake, especially on flop with no clubs? Thoughts?

PS: This was in WA. So actually it was 5-300 spread limit, but the $300 max bet didn't really factor into this hand.

Your hand has no showdown value, obviously. We would think that would lend itself to bluff, along with being heads up. However, he called the flop with the BB yet to act. The BB has only checked in a natural position, so he does not know if the BB is going to play or not. When he calls the turn, there are many hands and draws he can have, along with pairs + draws. You hold zero straight blockers and zero flush blockers. The top pair changed on the turn and you are betting on the smaller size throughout. I would have much rather tried to realize folds with a lager flop and turn bet. While the hand has zero SDV there is a good chance that he either picked up a flush draw on the turn or made 2 pair plus. I think you should have waved the flag in this spot.

As for your thought on triple barrelling, I think it is a mistake to lock your mind into just making a play. You should be open to ideas and information as it presents itself. Instead of just deciding on a play. The exact run out here has flushes, straights and 2 pair, which, I don't think you are getting him to dump on the end. Since we have bet small on flop and turn....not terrible to just give up here and sell a little advertisement if need be.

More flop
More turn
O/b shove river

Fold pre

Let me get this straight, you're expecting an OMC type to range your hand put you on a back-doored flush and fold?

I doubt that's what happened. More likely that he called you with a flush draw and folded when he missed.

This is not a great runout, as a lot of villain's hands have improved - T9s, 89s, 88, TT, plus the flop is a board where villain can flat sets often. Plus, OMC types view scare cards differently - the T and 8 are gonna be seen as blanks, even though they very much aren't.
Plus, the backdoor flush draw flop blocker is the lowest pair, so villain should have some pair combos that hit the flush
It is good in that only we have QJ/67 and villain doesn't. For that reason, if we're gonna bluff river, it's a jam; but I think we prefer hands that have the Ad in them, rather then what we have, which is a slight blocker to A5/A3 hands that are calling you light thinking they have some showdown value.

If I'm bluffing river it's going to be for 2x pot since we don't have much value to be betting. Since we can't do that I guess 300 is what we would have to use. Idk what the meta is on max bets on the river in spread limit games. Is it perceived as bluff or value generally?

All that being said I don't think this hand is a triple. I much prefer an over bet turn and then give up on the river. Yes we're at the bottom of our range but we block wheel draws and the flop is insanely dry. He already called twice and the board didn't change much on turn or river.

Flop cbet size I would also go larger

Look to bluff the turn on high disconnected cards in this scenario. Kings or queens. But on the Td turn, yuck. Time to give up. Like you said, bad card for you, good card for V. Idk. The more I think about it, without the back door flush draw on the flop this hand might be better suited as a check raise bluff on the flop. Hard to pick up equity on the turn, unless of course we make our hand. Maximize fold equity on the flop by check raising.