A better system is do away with income tax, and only have a resource tax including land, i.e. land value tax + sales tax + resources extracted from the earth tax. This tax should be high enough to guarantee healthcare, education, food, housing, water to every citizen.

A bigger safety net will get more people into college or careers so they'll be too busy to have kids for awhile, and will start a family in their 30s and then maybe only have two kids, if people are slaves to wage then they almost have to get married early to even afford a place to live and that often leads to children not long after.

A comfortable and happy society is a well balanced society and will have more natural population i.e. something close to repopulation quotient.

I don't have stats on this but maybe compare countries like the USA with Finland, and Denmark which often rank top 10 on 'happy countries' lists.