Blinds here were either 150/300 or 200/400 don't quite remember. Either way, all players in the hand had between 100-150 BBs or so.

Preflop action/villain descriptions

UTG+1 opens to a standard 2.2x or so open. This player was a middle-aged man. Had been playing a fairly TAG style, opening a pretty conservative range of hands preflop and playing pretty ABC after the flop. Did not cbet with high frequency; he would check and give up a lot on boards that should be favorable to his range.

HJ flats. This was a young American player. Had been staying pretty quiet up to this point. Opening a reasonable amount of hands but not showing a ton of aggression and not appearing to get out of line. Most of the pots we played together he just got out of the way. 2 examples that I remember.

Previous hand 1: He opens from HJ, I flat from BTN. J86r flop checks through and then he check folds to a 5 on the turn.

Previous hand 2:He opens from BTN. SB flats and I flat BB with 98s. Flop comes Ac9d8d. Checks to him, and he cbets. SB calls. I put in a pot-size raise and he folds.

Hero is in BTN with Qd8d. I decide to flat and take advantage of my position with a marginal hand against 2 players that weren't particularly aggressive. Also worth noting that the players in the blinds hadn't been 3betting me all tournament so wasn't worried about that.

Postflop action

3 players to the flop which comes TdTc9d giving me a straight flush draw

UTG+1 checks. HJ fires a half-pot bet. I flat and UTG+1 folds

Turn comes 7s. HJ checks and I check back.

River comes 8h. HJ checks. Action to hero.

Should I stab here or try and shown down my pair? Should I have been more aggressive at any point earlier in the hand?

I think you played it fine. I do think you should turn this into a bluff, as your showdown value is not all that much; it feels like Villain has a lot of 9x, and you block his flush draws.

Gotta bet turn with a low SDV, extremely high equity semibluff.

River is close, I lean towards xb because from your descriptions I don't see HJ having Q9s, K9s pre, A9s possible. Looks more likely to be AdQ/KQ type hand (though we do block those) that went with a one and done line otf. I doubt 76s stabs flop except for diamonds ofc, and A8s prob folds pre and/or barrels turn.
In summary I think you have enough SDV with 8x that xb is slightly preferable, and from a range POV assuming you xb turn with a few KQ, that will obv be a higher priority bluff here.

Preflop: Agree with flatting. Folding is also perfectly fine but if you feel like you can out play your opponents in position, go for it. Too weak of a hand to 3b an UTG open.

Flop: Agree with the flat. Not raising keeps both villains ranges wide. It also lets you represent a 10 I think.

Turn: When it checks to you I definitely think you can semi bluff in this spot. You have zero showdown value with a massive draw, seems like to perfect spot to bluff. Also, this board texture hits your preflop calling range well. By making this bluff I plan on betting again on all rivers.

River: As played I would just check back. I don't think hes folding anything stronger than your pair unless you make a very large bet.

This!

I agree with the general consensus here - we probably need to bet turn (and be prepared to barrel river to get all non-Tx to fold) and as played check back river. I think if we didn't have diamonds I could see using Qx (including Q8) as a bluff sometimes because it does block straights.

turn does seem like a good spot to bet. not that concerned about being check-raised since only one card in the deck gives us the unbeatable nuts and bf may extract you cheaply from an expensive RIO spot

however if we are called then i'm checking back that particular river. i don't have much 9x in V range pre other than T9s/99, we now beat some busted Adxd fd's, there's some JJ, QJs, AJdd and other stuff that got there. also don't discount the idea that V flopped gin and decided to take a cc cr trap line against a known aggressive opponent.

I ended up checking back and villain showed A9.

Think I should’ve bet the turn but was overly paranoid of getting x/raised off a strong draw.

Does checking back on the river here and then showing the hand help or hurt table image for later bluffs? Hero had great opportunity to bluff on turn and river but checked both times. For next situation -- maybe in bigger spot -- with this villain or anyone else at the table (especially given deep stacks and slow structure), does the check-back serve a useful long-term purpose?

your draw isn't that strong on a paired board. if you find a check-raise here, then your straight and flush draws drop significantly in value against a range dominated by trips and boats.

imo, it's worth cashing in the low equity straight flush draw in this spot for a high equity bluff.

it couldnt matter less.

It would be a very strange and unprofitable line for our villain to check turn with trips unless he puts us on exactly J8/86 or stronger trips. Full houses he should be betting 100% to build pot and extract value from worst made hands and draws.

The fact that this board is paired makes me like a turn bluff even more as we can rep the 10 after our opponent checks to us.