It's something I've been thinking about a long time. And everything you said, I agree with. Not to mention that in many households, women still do a lot more of the work (emotional and house-related). It's too much to handle and we don't know how to change that.
I would also point to the oft-quoted fact that the more anxious and unhappy we feel about the future, the less we tend to reproduce. That's the theory I'm working with at the moment, at least. It's dystopic to me, as a young woman, that we live in such uncertain times, are aware of the possibility of oncoming crisis (be it another pandemic, war, migration-related societal change in Europe or other such), yet are expected to still bring kids into that future. I'm stocking up supplies in case of a crisis, yet my body is supposed to feel safe enough to reproduce. that's nuts.
On the other hand, there's literally no better way to avoid that future than reproduction. If we stop reproducing, we might as well be culling the forces of good ourselves. What better way to ensure that your values, your goodness, and all these other gifts you hope to see preserved in the future, will be preserved?
RE: What caused the fertility decline? It's not smartphones. There is a much simpler explanation.