what I would suggest is to change the social structure of developing nations while at the same time introducing economical advancements steered toward cultural evolution.

here a few methods

education can help curve fertility rates,but how useful might that be in a male dominant society where forced marriage is still the norm?
the solution:introduce norm that empower freedom and independence, while shaming those infringing on those right as if there committing a taboo, and that it acceptable for couple to have no children at all

2. the senior population might want more grandchildren for various economical and personal reason. diminishing workforce,more retiree then economic supporters,old mcgutget family-owned farm need more child labor etc.

the solution: keep advancing technology to at least substitute for the difference in the job market while the population is in transition for a smaller demographic

3. especially in developing nations, the main reason why people procreate vigorously is to better there odds of passing there legacy and that one will be a success in a poverty stricken area, more children mean better their chances.

the solution: make the opposite true; offer programs for single-children (or that has only one biological sibling) that allow various resources to be accessible to them. I don’t think they would want 7 or 8 gosling in a polluted pond if they know that stopping at one will ensure they (under-tale reference) will become a geese thriving in a crystal clear lake (world)

WHAT DO YOU THINK?