[tangent128]

Even if you embrace a lower population target, you still have to deal with the billions of people already here.

Enough people want to leave a biological legacy, so halving population each generation seems optimistic. (yes, many people don't want children at all, but many that do also want their child to benefit from having a sibling. I will assume these numbers cancel out.)

So it will take centuries to get down to the "low millions" range, even longer if lifespan extension pans out.

Automation, meanwhile, is already happening, and the current wave is more likely to be felt in a span of decades rather than centuries. Unless you count "starving in the streets" as a "natural die off", redistribution is going to be needed regardless.