I would ALWAYS reraise turn in your shoes cca 3/4 pot and Villain almost always folds 2pairs/ sometimes calls w/ sets.. I dont find myself almost never in these spots OTR....

Played as played its easy call ( pot odds, size of bet, playing IP, etc etc etc)

What do you do against a reraise 300bb deep on the turn or a check-raise on a blank river. Or a big lead on a J/8/6 ? I actually folded the river, but regret not calling, as he might have worse for value for that size, as well as bluffs.

For me better question is : When he bets so small OTT, what does he value bet and what does he rep..? I think he would have bet more having NF, dont you think so? Or c/r maybe if he thinks you'll bet worse flush when he checks turn...

I dont say its  correct to reraise turn, but just because he bet so small I would go for it and put him to test.

Talking about preflop I guess the problem with this hand 300 bb deep is that it's hard for hero to stand big barrels even having a flush. Usually we will win a little and lose a lot to higher flush.

That's why I would definately 4b preflop (mb less than potsize). And of course call 5b.

As played I would reraise turn upto 2/3-3/4 pot.

Don't feel strongly about 4b or not with this hand deep. Think K/Q high flush players fine though without as you will either have an easy call-down if he barrels a ton or you will get to sd cheaper pretty often.

Results. Folded river and regret it as I think he can have worse for value.