insulation, then radiant barrier won't help that much.

Our current house has low e double pane tinted windows, radiant barrier, and extra insulation. Our electric bill was only $250 this past month. In our old house (built in 2004), our electric bill in July and August was 450-500 and the house was 1700 square feet smaller.

If the ac service guy says your unit is good then it most likely is. There's so many places where you can be losing air though (e.g. Windows, door strips, cheap insulation).

Also most houses in TX built pre-2008 were not built energy efficiently. It was around 2008 when radiant barrier and other energy efficiency items became standard on new homes here in TX.