In my mind, demographic decline shares a lot of similarities with climate change. These problems have really long feedback loops, which makes it hard for humans to deal with. Both are serious, but not directly extinction-level problems. Both involve a lot of value judgments (future beings vs current beings, what beings have value). Both are intractable at the individual level and require collective action to tackle. Both will require multi-pronged approaches to solve. I.e. There is no silver bullet.
Capitalism and socialism require economic growth, and demographic growth is a factor of economic growth. Maybe someday technology will decouple that relationship, but it's not today. Other economic systems may not be dependent on economic growth, but are they equipped to handle economic decline? Basically, my chief worry is elderly poverty worse than the Great Depression.
Like climate change, this is a spectrum thing. If we maintained current temperatures or our current population, most people won't notice. If we go from 8 billion to 1 billion people in a few generations, that will create a lot of human suffering.
Good policy on issues seemingly unrelated to fertility can help. Improving housing policy will improve household formation which will improve fertility. Healthcare is a huge concern for parents and prospective parents, so improving this will help too. Abundant clean energy will ease our worries about our children's future and likely improve fertility.
For demographic specific policies, immigration is zero-sum at a global level. For the US, it's a lever that will remain available as long as we're a desired immigration destination. I hope we maximize it. Political support for immigration depends on the rates of immigration. So higher fertility rates should support more immigration. Also, people of an age to have children in the US are already majority non-white. Those who want a specific ethnic majority in the US have already lost.
Industrialization, feminism, and liberalism have all impacted fertility rates. Industrialization has removed the internal economic benefit of children (cheap agricultural labor). Feminism has raised the opportunity costs of having children (the impact of children on women's careers). Liberalism has externalized the economic benefit of children (now even unrelated younger people will care for in your old age if you or your government can afford it). I remain a big fan of all three of these ideas. You'll have to tear my smart phone, the economic contributions of women, and social security out of my cold, dead hands.
But it does provide a blueprint forward. To improve fertility, we need to promote the non-economic values of children, reduce the opportunity cost of having children, and otherwise make sure that having children is not a losing proposition in society, so we can maintain society going forward.
At the individual level, I have no judgment. We all live in the context of our time. If you decide that having children is not worth it (whether financial cost, personal freedom, environmental cost, or something else), I get it. I struggle with those too, and it doesn't take much imagination for me to see how someone could forgo children.
At a societal level at a bare minimum, I think we should strive to enable people to meet their personal fertility goals. If that's 0, cool. If that's more than 0, we should be trying to provide economic opportunity, non-toxic mates, and everything else people need to meet those goals.
I firmly believe that for me raising my children is the most significant thing I'll do in my life. It's an 18+ year project that will cost hundreds of thousands of dollars. Selfishly, I wish society made it easier for me. I definitely wish society made it easier for my wife.