Don't forget collapsing testosterone rates: https://www.urologytimes.com/view/testosterone-levels-show-s...

I don't know what the explanation is, but I find your's implausible: "Only if the mothers in aggregate truly believe that their children will have good lives, then will they have them." I think that might be true in certain bubbles, but I don't think that explains why the fertility rate has collapsed just as much in Scandinavian countries that have the highest reported happiness ratings in the world.