Switching to acrylics and using a dual fan spray booth, vented outside, reduced/eliminated the paint smell to the point where my significant other allowed me to set up my modeling workshop in our spare room. (She also let me install a wall-to-wall display case in the dining room for finished models, so I am doubly blessed.) If space rather than smell is forcing you into the garage, then as you noted, some form of AC is a must to control the temperature and, more importantly, the humidity that can cause all sorts of issues with acrylics.
That said, insulating the garage door and the space above the garage will help your AC work less. We have an east facing garage door and the door insulation does help, but the 20 degrees figure is hyperbole at best. An additional option to consider would be the installation of reflective insulation/foil in the attic. This can be attached to the rafters so it would not affect the use of the area for storage. We have had this installed in our home in TV and previously up north, and the thermometer says it works.

Yes I insulated my West facing garage door. It was not too difficult and really made a difference.

Insulate your garage door and the ceiling above the garage and lanai. You will be surprised how much heat comes thru the metal garage doors, especially if the sun shines on them.

I recommend you hire someone to install the insulation or at least buy the precut insulation from RoMac that is designed for your specific garage doors. It is not terribly expensive!

I was amazed at the difference! At first I only insulated the double doors. The golf car door insulation was not available. You could feel the difference in the radiant heat from the two doors.

Our garage door had thin sheets of styrofoam which we removed and put in 1” think sheets and fit them all tightly in every space. It took some time but I think it helped. Our garage door faces west.

Garage Insulation
 

2 car 1 golf cart garage faces southwest.
Installed 2 garage door vents on opposite ends (one low and one high).
Insulated doors with 2 layers of reflectix.
Insulated roof rafters over garage with reflectix.
Have two attic access doors. Made and installed a screen on the smallest one.
Installed a gable vent fan.
Main concerns are radiant heat and air circulation.
Goal is to be within 5 degrees of outside ambient air temperature without using a/c or mini split.

I insulated my garage door with 1" rigid foam with foil sides. It works great. Yes it only slows the rate of heating up the garage, but overall it works. Example: open your garage door at night and let all the heat out. Then close the insulated door. Now in the morning when the sun hits the door, your garage stays cooler longer. It works the same way as your house.

Tom

Would a strong dehumidifier make a difference in the garage ?

The conditioned spaces in your house are insulated and sealed with a vapor barrier in the walls and ceiling. The vapor barrier prevents moisture from entering the house after it is removed by your air conditioner. So, unless you install a moisture barrier around the garage space, a dehumidifier would be very ineffective at reducing the relative humidity in the garage.

Cutting a hole in the drywall to install a vent fan to your attic destroys the fire barrier. Only safe option is to buy a fan that includes automatic fire damper.