Less humans will surely make more effect than any of other things you do. You can just adopt instead and raise them.
I would also accept "pay people to not have kids", similar to how some countries pay people to have kids [1] [2] [3] [4]. Not having kids has value, and hence those who choose not to should be compensated appropriately.
EDIT: @EForEndeavour: People who have children receive tax credits and benefits simply for having them (US centric) and their future impacts are externalities.
[1] https://qz.com/200728/what-countries-around-the-world-give-t...
[2] https://fr.wikipedia.org/wiki/Carte_famille_nombreuse
[3] https://www.france24.com/en/20130611-why-french-women-make-m...