Not the OP (though I am the GP), but by far the strongest variable correlated with massive drops in TFR is at least basic education of women.
So support local school systems with all might, which also means countering prejudice and religious bigotry of some locals. Once every girl has a chance to stay in school until at least 14 years of age, TFR will drop by more than half.
Start with military intervention / invasion. Then run the country, but not as a colony of exploitation, but instead of general good intentions with fairness, justice, etc. Then obviously you can set laws that limit birth, like one child per family or something.
Oh great let's instead let the world flounder forever and constantly civil wars in countries and endless amount of people fleeing, mass migration, endless resource consumption through endless amount of population boom, societies so backwards and abominable that you feel bad for the people living there.
That sounds so much better and so much more humane (sarcasm).