25% panel coverage is the breaking point before the return on investment becomes willy nilly. Car audio world does testing with this for resonance reduction, quite frequently and that is why I was prepared with said factoid.
If the remainder of the cabin is being rattled, then that is another problem entirely because you are inducing vibration everywhere due to the lack of reduction in your exhaust system.
There are weld on rubber isolators/vibe reducers that OEM's use to reduce what are probably exact frequency ranges that likely correspond to what would otherwise be hard tones inside/outside the cabin/body, but if I knew more of how they worked and in what
part of the sound spectrum, I could recommend some but I think the shear volume and intensity that you have being created directly beneath the chassis and reverberating off the ground below, is what will hold you back. They may just reduce the inherent tendency of the material to
resonate at a particular frequency.
You did mention wanting to wrap the whole system, but this is expensive due to the length and potential to tear it up due to road debris and immovable objects (speed bumps).
Get a muffler and add resonators inline, like two of them or the longest barrel you can fit down the middle trans tunnel area.
Start at the source, work less up top.
I am not sure why people build exhaust system with no muffler in the first place, as even road course tracks require them in all but the most remote of locations. What is the spec 94db?
Not harping on your personally, but life is so much better with a properly designed system. We like watching videos of cars with no muffler or resonators, and seeing them sometimes spit flames, but you can't live with them for any length of time. I'm also a fan of catalytic converters on properly tuned engines, so bear with me.