[OneEyedRobot]

>Once you answer those questions, is there a number of people you'd like to propose as optimal?

That's a good question. I'll take a shot at it even though you're not really asking a question but making a point.

Let's say that the one thing that people bring to the table is intelligence and the ability to design complex systems, it's something of an end-run on evolution.

How large a population do you need to build modern semiconductors, discover exoplanets, solve physics problems, do a passable version of the arts?

I would guess something on the order of 500M-1B which is roughly the number in 1800. You get most of the good and lose most of the bad if you're even slightly careful. The edge conditions of atomic war or a truly large nuclear incident still exist but the rest of our sins cover up well over time with that population.