My contribution to this topic is this:
Easily accessible contraception, followed by women’s education. This is working - it’s just something that takes time to work itself out, and most countries are at some stage or other in the demographic transition between high infant mortality, few people reaching old age, and a high pregnancy rate to keep the population stable and high life expectancy, low infant mortality, and a pregnancy rate that is observably low enough that the population will start falling again once the whole thing has worked itself out.