Easiest way to reduce population is to restrict immigration.

Straight-up. It works globally. People can only have so many people living in one spot. They won't grow much past that point. But if you let them spread out to other areas, they'll breed like rats, because like rats, humans are animals too. No immigration means the population in developed nations stays stable, and the population in undeveloped nations cycles up and down in controlled and predictable manners. Let the latter move to the former, and you push capacity limits in an upward direction every single year, with every single birth, seeking absolute maximum human capacity on Earth itself.