The blankets in the trunk is a worthwhile experiment.
So is silicone caulking cracks that might vibrate, or filling voids made of flimsy sheet metal.

Another worthwhile experiment is to use Vice-Grips to clamp different sized sockets inside the exhaust pipes at the outlet. This slightly increases the back-pressure and changes the speed of sound in the exhaust gas.

You already mentioned you can hear a difference in the drone was the engine coolant temp went up. That is also a speed of sound in hotter gas effect.

The pictures you have posted seem to show you have room to add closed end T- stubs. These are typically 26 to 18 inches long. Think of these as “blind alleys” the sound wave charges down, hits its head on the closed end, and dies. Start long and trim off length until the sound cancellation is greatest at your cruise RPM. The stub tubes can be bent, even coiled. They can also be the next smaller diameter pipe with just a small decrease in effectiveness.

You could also extend the length of your exhaust pipe past the critical 70 inch length somehow., like have the driver’s side exhaust exit on the passenger side, and vice versa.