Population size alone is not the determinant of sustainability. If that were so, krill, for one example, would be unsustainable. However, population size can make formerly sustainable behaviour unsustainable.

There is no solution to humans being a species. No species is more stupid than any other; they are all acting like species. And, OK, nature is not cruel, since it doesn't care one way or the other. But the way most creatures would end their days might be considered cruel to our civilised way of thinking.

As there is no solution, the best we can hope for is that the shit really hits the fan after we're dead (hopefully dying peacefully in our sleep).

I don't agree that awareness of inevitable death invariably leads to the cessation of all rational thought. This is partly because there is no such thing as rational thought; it only seems there is. Without free will there can be no rational thought process. But also because I think I'm thinking rationally yet I know my death is inevitable.

All of the approaches to our predicament that might help will have no impact, mainly because rational approaches would never be accepted by humans. In particular, only primitive living could be sustainable and that definitely is something people don't want.