We only use an extremely small sliver of that volume, the surface of the Earth and some of the space above the surface but that is so small it's hardly worth me

We only use an extremely small sliver of that volume, the surface of the Earth and some of the space above the surface but that is so small it's hardly worth mentioning. If the biosphere is the problem, then we should work to improve and expand the biosphere. Set aside land but also create new habitable land by greening deserts, expanding islands and pushing back shoreline to reclaim land. We've done this before, we're currently doing and we should do more of it. If habitable space for human occupation is the trouble then we should come up with novel ways to occupy the same valuable land with more people and provide more space to each person. We do this by constructing tall buildings, we should do more of that instead of having absurd limitations on building height in many areas. We should also innovate and build more things underground, the first obvious thing to move underground is transportation but there's no reason we couldn't locate buildings that consume a lot of square footage there too and in so doing free up the surface level e.g. sports arenas and convention centres. We could also innovate in the areas of mining and agriculture. Agriculture especially is currently very wasteful. It takes place in a completely uncontrolled environment that results in many inefficiencies. With sufficient power from a nuclear plant that activity could be moved indoors with a combination of natural and artificial light and heat enabling that land to be used year round for crops, increasing yields by extending daylight hours and reducing evaporation of water and wasted water that sinks too low into the ground or runs off which would greatly reduce water consumption and prevent wastage and side effects of fertilizers. It would also greatly reduce the need for pesticides while reducing loss of crops to pests. For mining there are proposed technologies such as extracting minerals from sea water or ocean bed mining.