> During two of my last major existential crises, for about two weeks following, I noticed a marked turn of my thoughts and feelings towards having (biological) children.

I have thought about this a lot over the years and your comment only enforces my opinion: we think more about procreation and having children when we are in more survivally stressful situation. That's why developed societies have less children and poor people have more: rich people or people with enough wealth to live comfortably don't have a lot of children because they don't face death every day and don't feel the need to spread their genes.