[blumpkin22]

I agree 100% about which cards to bet on sixth and when to take the free card instead. I'm not sure there's much room for debate there.

The river is more subtle. On the one hand, him betting on the river is suspicious for the reasons you mentioned. However, because of the action on the previous streets, he should put you on precisely (at best) split queens with a four flush, without two pair (before the river). It is mathematically more likely that you do not improve than you do improve. Thus, he can value bet two pair and fold to a raise if he is confident on the read that you will not bluff raise the river. If our opponent instead check/calls he is allowing himself to basically get owned, since Hero will not bluff the river after checking sixth street but will only value bet two pair or better.