I was going to suggest a porta-cool but those are anything but unobtrusive! (They sure work good though). Hope you find an elegant solution!

Are there any gas burning appliances in the garage (water heater, furnace)? If not, I would think a built in ventilation fan, whether through the roof or through a wall, would be helpful.

Is there adequate insulation on the underside of the roof?

Flat foam roof. How effective are the ventilation fans? Basically just bring it down to outside ambient temps? Correct?

Is it well insulated on the underside?

It wouldn't be a good idea to put in a ventilation fan on the wall or ceiling if there are gas appliances in the garage. It would be dangerous, the combustion products would go into the garage and not up the stack because of the backdraft pressure . But yes, I'd think one could keep the garage at around the outside temperature.

No gas. Part of the garage has a small crawl space. The other is just the foam on top. Not sure it’s insulated below

Can you get any hints or ideas from what your friends and neighbors have done for their garages? Unless you hang out in there, I'm not sure entering a hot garage and driving out is all that unusual in your area.

It’s more storing things in there and the radiant heat transmitting to the main house through common wall