how you would practically implement a top-down population control program that isn’t ultimately gonna try to sort people into who is & isn’t allowed to reproduce, or select for “the best/most fit/most healthy children” to be born?

Of course, that is very simple, (ignoring that you also smuggled in a top-down method which nobody mentioned). Proven positive interventions to reduce birth rates include:

•raising standard of living

•increasing access to birth control

•education of girls and women

•increasing wealth

•empowerment of women

There are other potential interventions that haven't really been tried because people insist on not talking about population - like honestly even a public education campaign about how maybe reducing the number of children could help the environment would probably shift some numbers as well. Honestly because we've made this such a hotbutton issue the actual environmental cost of a new human is basically never talked about and a lot of people have no idea that it could be an issue.

And look, the idea of genetically testing fetuses for genetic traits that might make a new human less happy is a touchy, controversial and complex topic that is obviously something close to your heart and I don't haven't thought much about and don't have much to contribute. I will, however, posit that it is completely unrelated to the topic of overpopulation.

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Can you actually give me a real explanation as to how you would practically implement a top-down population control program that isn’t ultimately gonna try to sort people into who is & isn’t allowed to reproduce, or select for “the best/most fit/most healthy children” to be born?

India, Thailand, Vietnam, Sri Lanka etc etc. Population control can be as simple as giving women contraception, education etc. That's what these countries did.