Natalism in the common parlance usually refers to government policies designed to make people want to have children. Practically speaking, that puts the cart before the horse. I favor natalism that starts with social norms and then is reflected in government policies.
Sustainable natalism is arranging society so that children are acceptable parts of the public sphere at all child ages. It’s making sure women aren’t broken and worn down by the stresses and strains of bearing and caring for little ones so that they have energy to pop out more than a couple and raise them to adulthood afterwards. It’s also about granting higher social status to married mothers and fathers, so that marriage is once again considered the correct place to bear and raise children in.
Sustainable natalism is people setting things up so that women feel that they can handle 3-6 kids, so that men can marry before age 30 because they have a good shot of being able to support three or four kids and a wife, and helping parents by being the real village, full of loving friendships and support. It’s discouraging atomic living and moving every couple of years for a job, it’s encouraging social norms that have extended family nearby. It’s remembering the value of cousins and siblings and aunts and uncles. It’s restoring healthy relations between single childless adults and children. It’s creating a social milieu that leads to grandkids and great-grandkids as a norm.
Tax credits are neat and stuff, but they won’t do the job. Society has to be oriented strongly towards children as a good in themselves, living the idea that they are a blessing, because modernity shows us that once any ethnicity or culture gets rich and bloated with cheap consumption, they get very uninterested in having children. Children are hard, even easy ones are hard. Without lots and lots of explicit support and status accorded to motherhood and fatherhood, people simply don’t bother.