Tangential point: You severely underestimate the impact of scale and specialization on our standard of living. There is no way we could the same variety of nice things at 500million people
To your tangential point: I think technological development will allow less and less people needed on the supply chain to sustain the same quality of life for society. I think this is clear from history and it only goes in one direction.
I'm aware there's people that defend more extreme views like boosting abortions or other dystopian measures. I don't believe in any of those, I think everyone should have the right to reproduce if they feel like it, I'd just rather it be an informed decision, but I wouldn't add any control measures for fear of abusive mis-use by the government later on.
Streaming TV and Education.
Educating women, in particular, is strongly correlated with population reduction across the globe.
Two generations of that and the world could be stabilising a billion and half below where we are today.