If OP was going to make a pot sized bet on the flop,should it include the BB's bet on the flop?. i.e would it be $1.25 or $2 ( for neatness) or should he be going 3x or 4x the BB's flop bet to make the draws (KQ,QT,KT and diamond flush draws) pay to make their draws. I don't think there will be an KK,QQ out there or they may well have reraised preflop,JJ AA are small possibilities as we already know the whereabouts of two of each,AA may be slow playing preflop and JJ may be worried about bigger pocket pair preflop so just called. Anyone with a big Ace is likely to call thinking they have the best kicker.
By the turn the BB likes his hand , hes lead out twice into the pre flop raiser and just been called down I'm putting him on a big ace or the diamond draw.As he's just been called he may think the others don't have an Ace.I think Hero should have raised to $3.50 which would have made the flush and str8 draws think whether they wanted to continue, opens up possibility of all in on the river , being able to consider whether to fold if the flush or str8 draws hit on the river, and possibly gets one of the opponents to fold so that its heads up on the river.