I can assure you that is not the way. Neither would it be the positive thing you think and expect. First of all, humanity tends to get proud of material things and the more they get, the more they want. So what you said:

The idea here is that if we are that small, we would require much fewer resources to live. Think about it, everything we would need would be small. Small houses, small cars, small amounts of food, a lot less energy, and so on.

That already happens in some parts of the world. Humanity doesn't need that many resources to survive. If the choice was sustainably first and money/capitalism second. We would have all the resources we need and always had them, the right question would be 'if everyone wants that' since is not profitable for everyone at the moment.

Another huge thing is humanity always try to seek another solution, instead of focus on the main task they currently have in hands. This 'solution' you posted about 'shrinking population' is the same thing has put humans on Mars. Don't try to avoid the mess by creating another one. Tackle the problem you have and that is always the right path to follow and inspire more people to follow.