and no, it’s not a financial thing
Over here in Estonia, we have observed that it is a financial thing. Research has shown that parental pay (the state pays X% of your previous salary for up to 475 days of being on parental leave - and as a sidenote I think the measure is wrong, because it means the rich and poor get different sums) encouraged people who already had children to have a second or third child.
But the measure had limits. It did not measurably encourage people to have their first child, and the birth rate continues to decline despite the measure.
I don't know the details because I'm not a parent. On personal notes, if suddenly, I discovered myself in a stable relationship and a hypothetical partner asked me if I want a child, I would reply "do I look like a big wheel? I can't afford that".
Hypothesis: today's young people know a bit more about finances than previous generations. They know that one should not have a child before one has stopped renting and bought a place to live in. This being impossible early on, decisions are delayed and the suitable time passes. Grandparents who could potentially help with child care start needing care themselves. Also, people increasingly live at distance from their parents, so grandparents' chances of helping with child care are reduced even if they aren't elderly yet.
As for relationships, research indicates that they break more often. Arguably the reason is that people don't want to remain in a badly functioning relationship, and haven't got the skills to keep it from breaking. (I don't even want to get started about dating sites - it's not profitable for dating sites if people find compatible partners. It's their customer disappearing and stopping to pay.)
Skills could be taught in school. Dating sites could be told to redesign themselves, accounting for sociological knowledge. Social security for parents should be effective enough to support a single parent who is renting their place of living, and should last long enough for a child to reach shool age. Such a policy should have constitutional guarantees (not changeable with 1 parliament). If this was done, population decline would slow over here. Not sure about France.