https://en.wikipedia.org/wiki/List_of_countries_by_total_fer... suggests Chad and Somalia and DR Congo are the most vibrant societies by that metric.

You might want to compare Singapore with a city like NYC or London, not with a territorial state. It's pretty normal around the world for cities to be replenished mainly by people moving in.

(Of course, to be fair you then also need to compare GDP per capita against other cities. And they usually do a lot better than territorial countries that include a lot of hinterland.)

What if you omit states that depend on welfare?

I never said a high TFR means a vibrant society. A low TFR is indisputably the slow death of a society.