The reason for birth rates declining seems pretty obvious to me, it costs $300,000 - $400,000 in the US to raise a child until the age of 18. Previously, having kids was economically beneficial, now it is not only not beneficial, it's extremely expensive.
I think the arguments about feminism/birth control/education/modern dating being the driver of the downward trend are incorrect, because we see very similar birthrates in countries that are much more patriarchal. Even Saudi Arabia has declined from 7 a few decades ago to replacement level as the economic model has changed, and it's still trending downwards.
Look at the countries with high birth rates. It makes economic sense to have children in these countries (with a couple of exceptions), because the state is not able to replace the economic role of children in people's lives.
Also, if you are going to say but in Sweden they have free childcare etc and they still have declining birthrates, this might make a small difference, but a reduction in cost is still not a benefit. They might pay less than US parents, but they still lose financially vs not having kids.
This is not a serious policy idea, but as a thought experiment imagine that everyone that had children got a yearly salary of $50k per child plus housing. It's hard to imagine birth rates staying the same if that was the case because children would provide economic benefit on the individual level.
Disclaimer: I'm not really interested in discussing whether it's a good/bad thing if the birth rate declines. The interesting part to me is the cause, and the cause seems very obvious.