over the decades ive played with many exhausts on many cars.... the one thing ive always found to improve things is the length of pipe after the rear muffler. having 6 inches of tail after the muffler makes it much quieter than having just 3 inches....
the other thing is having the tip too far back under the bumper, does a similar thing.
its not so easy to lengthen the output pipe these days with one piece stainless mufflers and tips...
and yes, stay away from wrap. agreed.
sound deadening such as dynamat can work but its much more expensive and heavier than just adding another glasspack muffler.
i would personally try packing cavities behind trim panels (bootlid, boot sides, parcel shelf) with quilt wadding as a cheap trial.