It's solipsism in the sense of monistic idealism: solipsism relativism to the one consciousness, and a denial of an external reality.

If "consciousness" is undefinable, how can this model have content? How can it compete with naturalism or materialism?

There are problems of skepticism for the naturalistic view that the brain causes consciousness. There are problems for all models, including yours or Hinduism's. It's a question of comparing the possible models and seeing which works best for certain purposes.