Reducing our agricultural output by 1/3 wouldn’t come close to making it sustainable, though it would certainly be an improvement. Fertilizer costs have been a big problem worldwide, so if using waste products were practical we would be doing it already. Shifting weather patterns from climate change are why a lot of those fields are fallow, and that’s only going to get worse.
Countries that use less than average resources are working far harder to use more resources than rich countries are working to use less, and I don’t see a plan to make that change. As individual choices go, no choice a person can make will reduce their impact more than having one less child than they otherwise would. We just don’t need 10 billion people on this planet.