Did 150 °C (300 °F) ruin my cream sauce, or is it just overcooked, or fine?
I think it would have been better to have kept the sauce separate and make it after the casserole has cooked.  White sauce needs the temperature reducing as soon as it starts to crack; indeed, lift it off the heat as soon as you feel this taking place. It is possible almost to thicken it completely from the heat already in the pan.