Here’s a few things to consider, but I’m hardly the person to give an authoritative list.
What are our quality of life targets?
We can support a crapton more people if we all go Amish. We gotta reduce growth to a global lottery system 30 years ago if we want everyone in the world to live like a median American.
This isn’t a one-size-fits-all-age answer, either. People need more resources as they get older, and contribute less work in return. An aging population means more economic stress on the younger population, and less economic output relative to each senior citizen means less access to medical care.
What are our sustainability targets?
Some things are getting bad faster than others, some things are closer to breaking points, etc.
How much do we want to bet on degrowth vs. innovation?
If we assume only tiny incremental improvements for centuries to come, then we’re preparing for something very different than if we’re trying to keep research investment steady or even accelerate progress on things like fusion, carbon removal, microplastics remediation, and power distribution and storage.
What policies are on or off the table?
Some philosophies say that limiting a person’s reproduction is categorically immoral, even if the predictable consequence is that everyone dies. Some TESCREAL dudes say we should use nukes cuz the ends justify the means.
How do we mobilize these policies?
We have lived experience that an aging population isn’t great for getting effective policy in place.
What about the political fallout?
Population change policies certainly won’t be done globally in lockstep, which means in order to stabilize local economies, there will be more immigration for places where the internal population growth is slowing/reversing. That can easily lead to xenophobia, which could destabilize everything. It’s hard to fight global climate change when you’re dealing with local fascism.
etc.
That’s why I can pretty much only reliably say “people who don’t want kids… not being forced to have kids… is an unambiguously good thing” and I can’t extend that to people who do want kids.