A sales tax as a general term on goods that have negative externalities. That produce pollution, have negative health impacts, use public infrastructure etc.. whole foods, homes at minimum should be exempt. I agree that the poor shouldn't bear the brunt of tax policy changes.
Yes tarrifs getting passed to the consumer is completely the point, to normalize for asymmetrical human rights across the globe. Fair trade, not free trade. Not isolationist either. An elegant way to implement would be based on a democracy index.
The aluminum example is a good one. The consumer in this case is the company importing aluminum. They can buy from an authoritarian country at a 2x tarrif (or whatever), or a democratic country with no tarrif.
But... more of a thought experiment, I think that would be the way from a humanist perspective. But the geopolitics are very challenging.
Lower population in of itself is a good thing.
It's the change that is disruptive and will cause suffering in ways that are unique to the suffering caused by over population.
As population growth slows, the younger generation needs to support more elderly. Which means we need some combination of:
Working population being more productive. Population making do with less.
However you approach it, there will be segments of the population that are very unhappy.