I find the whole Dyson sphere assumption rather odd. It could be said to be a kind of colonialist mindset insanely always trying to extract more. Why assume we will always need more and more energy? Perhaps our energy use will just become more and more efficient so we can do all we need with less energy. Or other tech beyond the basic idea of just harvesting solar energy may appear. There is already the concept of Zero point energy , the latent energy of the vacuum. Maybe that is the next level.

To argue that Dyson spheres are the only possible energy outcome of an advanced civilisation, and their seeming absence thus means there is no such civilisation seems pretty narrow thinking.