If everyone has 1/2 kids, the outcome is the same as having no kids, just with more years to get there. That’s Japan’s biggest problem right now. People are having kids. Tokyo is fairly kid friendly, and infrastructure/culture is there. But nobody wants to have 3 kids.

Simply not true. 2 is the replenishment rate. 3, is a 1.5 increase generation to generation. Our population is out of whack with the resource load. Your model is orders of magnitudes too simplistic.