The insulation is keeping the garage much warmer, especial y noticeable in the summer. You could cut a hole in the ceiling and add an insulated access door that you could open in the summer and close in the winter. Some of these doors are spring loaded and you could only open slightly when needed or leave open for the summer or warm months. This would allow the warm air in the garage to rise into the roof area. If the soffits are set up and venting properly, convection will take care of the rest. It will still be warm in the garage in the summer but at least you will have some air movement. Having a small vent at a lower area will also help, or you could leave your garage door open a bit at the bottom. You most likely don’t want to do that. You could add powered ventilation but this would be more costly and not really needed.