OP
approximately where do you live? Is the garage climate controlled? How hot will the summers be and how cold the winters? If you are going to have to open the garage doors to tolerate weather then it doesn’t matter what sound treatments you do. And even if you drive prosaic cars, leaving them outside will cause them to age faster. Squirrels and mice love to nest in engine blocks and chew on cables (I know several people that have to park outside that have encountered this). And then there is the most dangerous mammal of all, the car jacking homo sapien. Parking outside makes you a more vulnerable target.
In short, a terrible idea, imo.