Not GP nor an Icelander, but I literally just left Keflavik this morning, my first time in the country. Had a great stay there. My Airbnb host said that August to April is peak aurora borealis time, but May to August is the warmest weather - likely 50-60°F most days. He said 8-10 days for the ring road is good. Don't do much less than that or you'll be rushed.