Breaking Up (Planets) is Hard to Do
This Isaac Arthur video uses the analogy of a mountain. Living in a cave can be an adequate structure to live in but it utilizes a fraction of the mountain's mass. Breaking down the mountain and using the material to build rock homes allows for more livable area than the original cave.
There is a lot of raw material around the solar system not being used. Rather than colonizing the other planets the material making up the planets can be stripped away to build a swarm of habitats and solar energy collectors around the sun. All of the gases from the giant planets could be used for fusion and other industrial purposes for the countless habitats which could be built.
The whole process of dismantling the solar system would take thousands of years and a lot of energy. The video gets into gravitational binding energy and harnessing the Sun for energy and material.