I think Dyson Spheres are a stupid idea, a civilization with the power to process that ammount of raw material wouldnt be loooking for photons or heat as an energy source , they would be gathering energy from quantum fields or some   other type of undiscovered or still unimaginable type of technology, available to us in maybe  100k years after  the AI singularity, when our current level of technological development will look like the dinosaur era to us or worse.