Best method to cool a garage?
I had a grey tin shed that was getting very hot in the summer.   The cheapest and quickest fix was to paint the sides and roof side that faced the sunlight, with some random white paint I had laying about.  It wasn't a quality job, but in my case it was around the back and not visible.
Decreasing the insolation is a good start, is silent, and needs no ongoing costs like power.
Then you can start exploring more costly solutions like active cooling.