What is the relationship between ethics and meta-ethics in this framework? You say that the goal is for human beings to “flourish”, “thrive”, and “lead a successful life”, but are those objective criteria? Individuals and cultures differ in the type of life they would consider successful, and the type of society they would consider thriving. Are they mistaken? Will a study of cultures that disagree on these points reveal that some are objectively correct (or more correct)?
I’ve written previously about the evolutionary function of morality being important to understanding of morality. I was reminded of it by @formeruser12 (I believe as a paraphrase of @PerpetuallyCurious’s point), that “morality [is] an evolutionary trait… that can be quantified and studied”. I agree with that phrasing, though I’m not sure that studying human societies is necessarily the right place to look for the evolved role of morality.
And the answer there will be meta-ethical: morality and the good are defined by their evolutionary role in shaping humanity. That sets something like reproduction/perpetuation of the species as the basis of the good.
From there we can look to societies to see what has worked, or what is likely to work going forward. But they are distinct questions: what to do vs. how to do it.
The other thought I had was whether ‘objective’ implies ‘unique’ for you. Consider e.g. Sam Harris’ Moral Landscape, which similarly argues for using intuitive ideas like human flourishing as a starting point for an empirical evaluation of moral systems. But he acknowledges that the answers may not be unique, that there may be multiple systems that equally well achieve the goal of human flourishing (or within the margin of error). These are the ‘peaks’ on the moral landscape.
Another argument I’ve made that relates to multiple realization and your goal in this thread is that society may be best off when individuals follow different moral systems, just as ecosystems are often healthier when species employ different strategies and occupy different niches. I don’t think the point conflicts with anything you’ve argued here, but pluralism is sometimes seen as in tension with truth, but shouldn’t be when we’re looking instead for what works.