One critical possibility was unmentioned. There seems to be a very strong argument that it's advertising that is driving declines in fertility rates. The entire point of the modern advertising machine is to generate endless desires and wants that one might not otherwise have, always chasing the next product - basically to create consumerist societies. The most common reason middle class+ people
say they aren't having children is because they don't have enough money, but it's simply untrue.
In most of every society, certainly including the US, there are plenty of people having children with far less than the incomes of these people that claim to be unable to afford it. So what they really mean is that they would rather spend their money on other things. But this is extremely unnatural. Our bodies and minds are heavily evolved to wanting to procreate - it takes a very strong force to fight back against the biological clock and it seems that advertising is just that force.
I think evidence for this is widespread. Many places like Thailand have low education, low income, and low fertility (on par with Japan now). The classic explanations for declining fertility would predict high fertility in such a place. But what Thailand has is an exceptionally high degree of consumerism. And vice versa this also explains why religion often correlates strongly with fertility.
There are varying views on sexuality, education, equality of the sexes, and so on - but most tend to offer a life purpose that supplants any meaningful role consumerism could have. Another bit of evidence is that fertility rates among the extremely rich are also extremely high. Again when one has billions of dollars you can't really be a consumerist because at that point there is basically nothing one could not buy, so you need to look elsewhere for meaning and fulfillment.