Your first pic. Your lawn looks very dry. Are you watering it?
Wow. You're watering 3 inches a week? I'm surprised by this. I'm watering less than this on what I had assumed was more sandy soil and heat stress hasn't been an issue.I water every other day, 0.5 in. each time, and that's just enough to keep it green. Any more water at a time goes through the soil and disappears into the sand. The same was true when I mowed higher.