Fixing the birthrate is pretty simple in theory. The government needs to meet the needs of people having kids. The details are a little more complicated.

In order to have more kids in their 20's people need:

A higher income in their 20's. If they work full time they deserve to be able to afford a 3 bedroom place, food, etc..

A place to live - Build affordable housing that people can own and build a life. These need to be 3-4 bedroom places that one income can cover.

Medical care: free quality medical care to cover little things like birth cost and the doctor visits a child needs.

Time: Hard to make babies when you are working 60+ hours a week. Mandatory 40 or less work week. 2 months of vacation every year.

Childcare - Free or heavily subsidized childcare for working parents. Currently childcare for 2 children is more than the net average income for one person in many areas. Earlier retirement programs are also highly effective.

Quality schools and education: ban private schools, invest heavily in public schools increasing teacher wages and requirements, reducing classroom sizes, and providing quality educational material. Free college and trade schooling as well.

Hope: Stop fucking up the planet for temporary gains. If we started to reverse our environmental damaging behaviors more people would be willing to have kids