Sounds like a good place for a whole house fan. Placed horizontally in the ceiling of your garage, you have the ability to switch it on with a thermostat or manually and pull massive amounts of cooler outside air into your garage. If it‘s 100 degrees outside and you pull a hot vehicle into your garage and the air temp inside gets hotter than 100, the fan goes on and pulls 100 degrees outside air in to cool off the vehicle. So the garage stays no hotter than outside air. You will need louvers or open windows to let air in. The big fan can’t exhaust hot air from the garage into the attic without a clear path for the air to flow out of the attic.
I have a 2000 sq. ft. house with a 24 inch whole house fan with a 2 speed motor capable of moving 5000-9000 cu. ft. of air a minute. If only one window is open, the wind is strong! You probably don’t need one that powerful. Get a knowledgeable person to size it for you and tell you how many square feet of eave or gable venting you’ll need for the attic.
OR… just spend more money and install a big mini split air conditioner in your garage and be even more comfortable.