Stack sizes aren't really relevant in this hand.

SB is a loose/aggressive player but balances his large river bluff frequency reasonably well making it somewhat hard to play against him when he's playing good.

Preflop:

Playing 6 handed. UTG limps and as he does so flashes what is either the 4h or 5h. I'm confident he doesn't know I've seen it nor has anyone else seen it. Because of this I decide to raise the button to $25 no matter what I have. I happen to have the 4d6d. SB calls, UTG calls.

Flop (Pot $75): Js9d5c

Everyone checks. I thought that I had a decent c-betting spot because of how often I can barrel the turn when I pick up equity. However, my history with the SB tells me he isn't folding this flop often and I'm not expecting UTG to fold a pair on this board either.

Turn ($75): Js9d5c8d

SB bets $40, UTG folds, Hero calls.

River ($155): Js9d5c8dAd

SB cuts out 6 green chips and 8 red chips and shuffles them repeatedly in the same pile mixing up the colors, then splashes them into the pot making the bet $190. Hero???

Stack sizes are always relevant...

My first question is what was your thought process on the turn? You picked up a gutshot and a flush draw here. Did you consider raising? What was your plan after you decided to just call the turn for the river if you caught a diamond and he bet again? If he checked? If you didn't catch a diamond and he bet or checked?

On the river your back-doored low flush is probably good, especially since you know he doesn't have the A and you've seen him bluff the river well throughout your session. But I'm not raising here.

I felt like stack sizes weren't relevant because I never thought raising at any point of this hand was a viable option, although i suppose i could raise the turn. I'm not really repping anything though...

On the turn i called because I thought I had decent implied odds and actual equity against most of my opponents holdings. However, the overbet on the river made me start to think that I wasn't good.

Do you think Villain can be value betting worse here? If so, how much worse?

You're basically at the top of your range, since I assume you can't show up with any of the bigger flushes because of your flop check. Villain may not even think you can have a flush at all between your UTG+1 raise and your flop check. I doubt you ever show up with QT here for all the same reasons.

Given that and Villain description, I don't see how you can find a fold here. I'd expect to be beating a straight, a set, 2pr, a bluff w/e... lots of stuff... plenty of the time if Villain is capable of VBing anything other than a flush and/or bluffing.

Stack sizes are very relevant. You have two ways to win. Outplay your opponents or make the best hand. Against a short stacks you'll prolly only win by making the best hand. UTG has at least one card that helps you, cutting down on the odds of making the best hand and semi-bluff opportunities. A good LAG should be putting pressure on you with such a draw heavy flop.

fold pre

as played call river

Stack sizes are relevant. I very well would keep initiative by betting flop HP if enough money behind 100bb+.

Call river with no thought.

the fact that he PRE-selected an overbet amount of chips and the fact that he had to "wait and think" for a while before putting in the 190 shows EXTREME weakness to me.

Raise this bet.
my read is that the villain spiked an ace, but he's worried about why you called $40 on the turn and he thinks you might have AJ.
This is effectively his method of using a blocking bet because you're not going to raise it unless you have something much stronger and you won't call unless you actually do have 2 pairs.

If you get raised after this, then you can only be facing Q10 and you were beat all along.

However i highly doubt this

Easy call. Very hard for villain to put you on back door flush.

Result?

Called and was shown JdTd. was not surprised at all to see him have a flush, specifically with the Td in it

hmm, so he flopped top pair OK kicker and it checked around

On the turn the 8 gave him an open ended straight flush draw, and he bet $40 into $75

On the river he makes the 3rd-nut flush and bets $190 into $155. I guess it all makes sense!

I still probably would have called in your shoes.

Would have been interesting if an off-suit 7 came on the river instead.