AIGHT HERE WE GO

The strongest predictor/corollary for birth rates is not spending on sex ed or contraceptives, not price per carbohydrate or anything like that- the strongest predictor is wealth of nation divided per person, oft measured as gdp per capita. The strongest predictor across time and the largest samples is wealth- the richer a nation gets its birth rate always begins dropping. The effect is differential just as the gdp growth of nations is differential but the effect is always the same- \^money, vbabies.

Lots of theories as to why and there are probably factors that get in the way of this a little bit ranging from cultural to economic but the strongest and most accepted theory is urbanization, that is more people living in cities where all the wealth is generated and out of the countryside. In largely ruralized societies you have lots of kids- each kids is an extra hand for labor and also the infant mortality rate tends to be high so you need to try and get ahead of that- kids are a net asset. In the city jobs tend to be a lot more clerical than manual and there's more attention on the kids- each one of them quickly becomes a net liability both because they can't help with the job you're likely to have, (in addition to it likely being illegal to make them do so,) and the pesky availability of medicine, resources and medical expertise concentrated in the city means that infant mortality is decimated making each kid annoyingly long-lived. There's natural incentives to having less kids in urbanized living.

This is not helped by the weakening of real purchasing power, as you mentioned- people are also less inclined to have kids when they can barely cover rent, don't get me wrong- but the predominant large scale predictor is gdp and it's happening everywhere. The places that experienced rapid onset urbanization and economic development in the postwar period like japan and korea became the tip of the spear in this phenomenon but there are signs even parts of sub-saharan africa is experiencing this in its more developed areas like Botswana. (This is also, funnily enough, the main impetus originally behind China's infamous 'one-child' policy- it was putting the cart before the horse in the sense that if you exhibit the side effects of economically developed urbanized nations you could make an economically developed urbanized nation appear- this is now, like many things from the Great Leap Forward, biting them in the ass).

Point is at broadest scales the strongest trend is economic development inversely impacting birth rates, catastrophic as the economy seems from the layman's perspective. It's not clear that there is a lot that can buck the trend.

It might be helped by increasing the real purchasing power of the middle class, making people able to more afford kids. It might be helped by applying antidotes to liberal propaganda that deconstruct the nuclear family a la Shinzo Abe's ghost. I don't know.

But these are unlikely to buck or halt an otherwise global and consistent trend.