>>520546820 (OP)
the problem is that Japan's (and America's) biggest cities are sucking in the vast majority of the more rural youth population creating death cycles in those towns that would otherwise be the main contributors to high TFR.
In a healthier society, towns can sustain themselves and cities are more like large community tents for those towns. If towns are caught in death cycles, people are just going to move to the city to survive. And with the increased fighting over resources in the city (ESPECIALLY land and mates but also money and food), TFR is going to start tanking. It's just a numbers game.
I don't really know how you fix it other than letting the rural communities die for now until both them and the cities are small enough that people can start expanding outward again.