Sound diffusion panels--do they reduce brightness?

IME no. Looking at your room, your speakers and seating position are well away from room boundaries. I would look to a change in amp or cables.

Doors are closed behind sofa now. Amp and cables just upgraded

If your room is bright, changing amps and/or cables is not a solution. Although you have bass traps and an open space behind the listening position (all help), your room still has a huge amount of reflective hard surfaces, especially at ear level and above. Adding wide-band absorbers at the first reflection points is a great first step. If that step is subjectively effective, consider more.

Kal