I don’t have all the answers, it’s like saying I want candy and not knowing how to make it.
That said, maybe something like a tax on children, free contraceptives, free sterilization, free abortion. Pay people when they reach 45 if they don’t have kids. Robot caregivers for elder care in a decreasing population.
I have a setting and characters but no story. And I have stuff that could be part of a story, but I don’t know how to put it together. Like an arranged marriage, and jealousy between someone born to power and someone who fought to earn it. I just don’t know how to make a story that fits all of them in there.