Not knowing how your garage is constructed, if it does not have a ridge vent or soffit vents or a vent in either end if it has gables. I would not prop the door open any farther than the width of a brick, about 4 inches. If there’s venting, your doors are not real tight and it’s not sealed up and insulated, I’d say you probably already have adequate ventilation. That’s my opinion. Kind of hard to tell without being there.
Strike while the iron is hot. Live every day as if it were your last and one day you will be right.