Great to see more people focusing on this issue! I've written about this topic before, and one of the most compelling/feasible solution areas seems to be living space.

In contrast, Katherine Boyle presents a theory that aims to attack one of the core underlying causes — urbanization. She believes that the collapse in the fertility rate stems from the decline of multi-generational households as 20th century Americans moved into cities. Having extended family members to help with child rearing makes a big difference, but this requires more space. This is where, she suggests, permitting remote work from rustic locations can boost fertility rates — a rural renaissance powered by Starlink. Upon deeper analysis, Katherine’s focus on physical space being a primary driver becomes more compelling. Evidence suggests that as home prices rise, fertility falls. In the United Kingdom, a 10% increase in housing costs led to a 1.3% decline in birth rates. Building more homes may boost fertility.

I haven't found research to back this theory up, but anecdotally it seems that the real factor is access to living space, and cost itself is just correlated with that. We don't need more high-density housing, but more communities.