Is not employment rate vs. population one valid metric, for instance?

It is not. Is there any correlation between unemployment and overall population across time? I don't think so. Is there any correlation between local population density and local unemployment? I don't think so. Is the unemployment in Hong Kong hugely greater than in Mongolia or Greenland?

cost of various foodstuffs vs income?

As with unemployment, look at this criterion over the last few centuries. Even during the XX century I believe the percentage of income spent on food has been steadily dropping in the developed countries.

But we don't know enough about future technology and sociology to say for certain.

It's funny how the proponents of the overpopulation thesis have absolutely no problems with linearly extending resource consumption lines far into the future but can't say anything about the future technology and so conveniently assume that it won't change.

My goal is for individuals to have more wealth and access to more resources.

So, that's pretty mainstream. Would you be fine with calling it the total economic wealth of the world?

if you were to disperse the population of India and China around the world

Let's stick to reality.