Shove obv.

on such a lock down board i don't mind your tiny sizing

The small bet is certainly defensible, imo, considering we have both the nuts and 2nd nuts covered on the locked down board.

However, if we posit that the BB has an AA combo a high % of the time due to the preflop action, could we then assume they are calling at least one street on the A-hi board no matter what? Which would then mean as an exploitative play betting bigger in this scenario being the better play?

With this, we still have the BN left to act and a UTG opener still in the mix, so any bet will be perceived as a pretty strong holding, even one on the smaller size compared to pot.

Lastly, what hand are we keeping on the leash that wouldn't call a larger bet? Maybe a TT? Or someone's secondary cards that happen to be very low diamonds? Both seem unlikely.

Slowly talking through the hand, I feel like a bigger bet is better, like 2/3 to 3/4.

Heads up is another story and you could definitely dabble in small bets to open up your stabbing potential in a larger strat.

I agree with the poster above. You're looking to get called by opponents drawing to a boat, and opponents that are bluffcatching.

A bigger bet has the benefit of looking more bluffy or as if you're building a pot to make a big bet at, and giving a set less of an immediately good price.

I do favor a bigger bet in the 2/3-3/4pot range. That being said, it's a polarized board, the smaller bet sizing is more likely to get called light (low flush value calls, or drawing to boat up 2-pair), or looks like you're betting for thin value and could potentially induce bluff raise (but this Is less likely in low stakes, bluffing without a high flush card blocker is only likely from savvy players that have a strong reason to believe you're betting thin value). In a slightly different scenario where sets are less likely (like same prefop action and flopping a nut flush on 6d2d3d board), making the less than 1/3pot bet may be optimal. I think it's a defensible bet.

Villain's less than 1/4pot river bet is pretty perfect, as hero's range is mostly flushes and villain's line has very bluffs.

Sorry for getting off topic.

I think that calling his river bet is actually only mistake you made in this hand.

The river bet looks boat heavy, but calling getting 5/1 figuring that it's often a boat but worth calling for the times it's a blocking bet is reasonable. I wouldn't fault someone for calling or folding there. If the bet were $12 I think it's a mistake to call. Still enticing to call, but it's less likely to be a blocking bet at that size and lays a worse price so you need to be right more often.