Imagine a policy where kids give 10% of their earnings to their parents.
It incentivizes parents to have more kids and push their education. And it funds their retirement.
To keep children from becoming pure financial instruments, and to keep the genetic lottery alive, you blend it 50/50 with a public pension pool. And it only works where education is affordable and roughly equal.
It's not clean. But on paper, it holds.
East Asia indirectly runs this exact system (filial duty, children as retirement plan) and birthrates there are the lowest on Earth.
Maybe no single policy moves birthrates at all.
If so, it will solve itself with a shift in the collective consciousness.