what about Sam Harris' idea that human wellbeing is the end…

Human wellbeing is a rather vague idea. Some people would argue that humanity's well-being depends on drastically lowering the population. I'd say protect individual rights and then it's up to every individual what level of wellbeing they can achieve. Trying to create wellbeing for all sounds like a recipe for disaster, starvation and mass-murder.

I think he would say we can measure it by life span, the health of the population, the psychological health and feeling of autonomy, maybe even wealth

Utilitarianism - and that's what this seems to be - is exactly one of the devouring mother concepts: "Why don't WE just do X and that would create the greatest well-being for all!" It's the opposite of actual etgics/morality, which demand that YOU respect individual rights instead of violating them for a supposed "greater good" in some kind of messianic delusion. The "sheriff scenario" is a good way to understand the problem with this kind of pseudo morality.

yea, I think the problem is it's extremely difficult to know how to actually achieve these things in a top down manner... we have to respect complexity and let local agents make the best decisions for themselves as they are also principles privy to the most real-time and relevant knowledge

Yes. There is a messianic complex in there and a prophetic complex. But people are neither messiahs nor prophets and power turns them into demons, not gods.