I am a shed, garage, and patio builder in Australia.

Vents will help but insulation is far better all year round. I assume this is a timber stud wall garage with cladding on the outside.

If you can induce a natural flow with solar passive design principles it will likely be far more efficient and comfortable than trying to force air with a fan. You want to try to have an inlet vent on the cool side down lower and an outlet vent on the hot side up high which should be the south west for the US. This will natural suck cool air through your garage.

Adding a thermal insulation layer will be far more effective against the heat. I use a product call aircell a lot. It's kinda like bubble wrap, two strong and durable foiled layers with bubbles sandwiched between. It has a very high insulation rating, and it's very easy to work with, cuts easily with a sharp knife and will hold with most fasteners or glue. But it's not cheap.

Sorry to be that guy but the doors cant be closed as they dont have a jamb. They are only hung off posts. Just because you are facing the broad side of a door doesnt mean it is closed.

When is a door not a door?

When it's ajar.