Yes I misphrased, I wanted to say if he folds less than 66%. This ofc depends on his on turn-calling range which is a street many people can tighten up considerably. Not fishes/bad players necessarily, but they also keep calling too much on rivers. Your description of his player type kinda does not match (my) experience of turn/river tendencies. If he's a fish, he'll unlikely suddenly fold more than 1/3 of his range without texture shift against a medium-sized bet. Fishes do not study solver, this means they have no funneling down with their ranges street by street. Bad means sticky and emotional attachment to the hand. Hero's story of having a straight ott does not change otr. Even bad players understand you are not repping bottom2pair anymore. So his thought will be (rightfully so), "either he has a straight or he has nothing" and "if he has nothing my bluffcatcher will win".
Furthermore if you really want to bet despite all I have said, it makes sense to pot the river to make it harder for him to actually call the hand he'd like to see a showdown with. If he had a busted side-draw he'll fold no matter what and with nuts he is not folding ever so the end-result would be the same with those inelastic subcategories, we would aim for the medium-strength SDV hands, primarily (top-paired)2pairs to find a fold. Your range is polarized (nutstraight or bluff). Not betting big is fairly suboptimum meta stuff he probably does not even understand. He sees the smallish/cheap bet and is more likely to call.
I can ask you one more time what you mean with "bad". If he is extremely bad, when passive he is basically a callingstation and will just call you down thus no fold-equity. If he is calling too much early but overfolds at one street in particular, he is what I call "weak-tight" or "fit-fold". Against such a player his turn-range could be stronger than you might have assessed. This more experienced player knows you already rep the straight on the turn and he won't call very loose planning to just fold any river whatsoever, his flushdraw portion that'd justify that resulting from bdfd's is comparably not that big (you also block it). Against this type I'd quote:
Loose players can have it more often than you think:
...
and
Don't Blow Yourself up Against Overfolding Players:
...
Player types where bluffing river wide generally makes sense to me:
A (weak) loose-aggressive regular who's exploiting other regulars river inhibition-thresholds by defending turns wide and overfolds to river bets (over-realization strategy).
Another tight-aggressive regular who is in a counter/-adjustment phase where Hero's testing out the skill-level and willingness to increase variance ventilating up a new street.
You need players capable of folding to bluff them. Since a nits' range is superstrong when flop gets called besides fit-fold players this leaves lags and tags. For the latter you'd need to know how much % you're bluffing though when turning your specific combo into a bluff, otherwise it's kinda speculative. It's hard for me to recognize, what specific characteristics a 8♥:7♣:6♥:5♠: combo holds for you to say: I want to add this unfiltered air into my bluff-range so I have an x% amount of bluff-freq. First you should check out how many :Kx:-nutblockers without SDV you have there and if the amount of it is sufficient to fill up your desired bluff-portion. Sometimes it's possible to have a higher freq. of a single
than :8x:, :7x:,
and
combined. Secondly you want to add hands that unblock the flushdraw, as it is part of villains turn-calling range with a more weak SDV in general. To fine-tune additional hands, you'd watch out for key cards and what frequency they make up, Perhaps solver still prefers using certain pair-blockers to bluff here for whatever reasons, idk. I would not recommend adding random bluffs when there's no hint that the situation is auto-profit or at least somewhat likely to be profitable.
PS: I removed the quotes since the have a copyright. FOr more info check out "How to beat the Fish" written by Cory Mikesell.