I don't think we need population growth for humanity to continue prospering as it is.

As I see it, the issue is more that the purpose of prosperity is to enable human flourishing. Less people, means less flourishing (other things equal).

Not if AI can solve longevity and humans can live 200 years.

This is not possible, you have to look at that what we call 'AI'. The word intelligence in that context is not equivalent to the context of intelligence of mammals. There were some discussions or suggestions to call these models not AI but PP (probability programs). And AI should only be used in the context of a real AGI (artifical general intelligence). Furthermore, it is absolutely unclear whether it is even possible to develop AGIs. There are no programmers who can speak in this area without having the same in-depth knowledge in the areas of psychology and biochemistry. At the same time, a psychology and/or biochemist needs appropriate programming skills, although it actually goes far beyond simple programming to be precise. Unfortunately, there are very few people like that in the world. It remains to be seen what results current research will deliver over time.

Something is wrong with the world if it is becoming too costly to have kids.

I know its a bit scifi out there, and i'm not sure this Ai we see now will get to this point

But sometimes I think that if Ai does get to the general AI point it's actually the collective thinking of humanity, and it extends it beyond the limitations of the biological, if we're merely data, we can dematerialse the human experience and move beyond the current limitations.

We could transport that data to new worlds, reform it there, start again, to expand the pursuit of humanity to a wider universe with more resources to keep on exploring

This hinges on a good definition of civilization level prosperity.

I think humans will be happier in the company of their own kind, prospering with their own kind, but civilization prospering might not require humans be maximally happy.