My river sizing here with made hands and bluffs would be 45% - 75% pot depending on their image of me and tendencies of the players. At 75% we are giving villains 3:7 on a call (0.75%/(1+0.75+0.75)=0.30 = 30% = 3:7). I would rarely have bluffs here though I would have a lot of nut straights and full houses wanting to get called by smaller straights / bottom full and I would tailor my sizing based on what I think their ranges are for calling. I do not have anything that mathematically supports this sizing other than my own tendencies.

This is fundamentally a complex question. I do not have the software for it but it is out there... we would need to break down their ranges work out what they fold and what they call with after taking a line like this in a hand, then look at your ranges and work out a 70% value range and then fit in 30% of bluffs (or 66% value 33% bluffs for a pot bet at 2:1 to call), then work out what our bet sizing should be to get folds and calls in the right ratios. I think largely we are wanting to fold out hands like 67T8, TT59, and KKT8. It might be a bit ambitious to want to put both villains on hands that will share outs in order to make it look more correct to bluff this spot, and likely to fold out these hands we don't need to be potting.

Leading is a good option on the flop, but you definitely cannot check call this spot. After the turn checks through and you holding 64, I like your decision to bet full pot. You should do it with your boats, air, and can even do it with the 1 card straight to get others off a chop (particularly good if you have blockers to full houses).

Personally I don't think this is a good spot to bluff, the risk/reward isn't good, and the line you've took doesn't make much sense. Pricing out hands you want calling if you have it, the whole thing looks odd. It worked out, but I do think it needs to be a smaller river bluff