Hand is played at 20NL and SB and BB are both unknown players. BB in my mind is showing incredible strength and I feel like my flush is just a bluff catcher at this point or is it too nitty not raising?

Preflop
Hero BTN Qh9h 50c (2.5bb)
Both SB & BB call.

Flop ($1.50) 7h 5s 3h
SB check
BB donks 90c.
Hero calls
SB calls.

Turn ($4.20) 7h 5s 3h 8h
BB barrels $2.77 (There is a 2/3 pot button players use often)
Hero calls
SB calls.

Here I am starting to worry as he barrels into 2 people and SB over calls as well. I think BB has all AhXh and KhXh in range as he is only calling 1.5bb, so lots of suited hands in range.

River ($12.51) 7h 5s 3h 8h 2c
BB bets $8.25
Hero Qh9h calls
SB folds JhJc.
BB shows Ah8s
Hero wins Qh9h

In the future it's easy adjustment seeing how light this guy barrels MW. In real time though, he was unknown. Other question is, if they check turn what sizing would you use? If they check river what sizing would you use?