Just wondering, have you added as much insulation to your attic and any other spaces that add heat gain during the day.
I am in NH. SO, I am more concerned about heating in the winter than cooling in the summer.
I am also living in a 1972 house. I added a foot of bonded cellulose insulation to my attic. I caulked around all windows and exterior doors. I even took off all of the electrical outlet plates on exterior walls and foamed/caulked around those. I replaced 3 of the 5 exterior doors entirely. The two remaining doors, I replaced the weather stripping. lastly, we added insulating curtains to the sliding glass doors and all bedroom windows.
The other major purchase was a wood pellet insert I installed when Obama let oil get up to $150/barrel. My furnace burns heating oil.
The biggest bang for the buck was the attic insulation. All the remaining items contributed small amounts but cumulatively they reduced my energy consumption.
Obviously, your goal would be to reduce heat gain in the summer months. However, insulation works both ways, heat loss and gain.
I have noticed since I did all of the above that my pellet usage is now down from the first winter. It does not take as much energy to keep my house warm.
I put heavy insulation between the floors over my garage (a built-in garage in my bottom floor underneath my kitchen) since my garage is practically outside air even when the garage doors are closed. My wife and I also went around the house like you said and caulked up around the windows. I had already replaced the outside doors a few years ago anyway and added the rubber felt gaskets around the doors.