Nice review.
As an accomplished professional installer; and trained Audio Engineer, I can add a few tips.
Always work on the passenger door first. If you break something, this door is used less than the driver's door. Waiting for a replacement part will have less impact; plus even cosmetic damage is further away from the owner!
Work on the door with the window up. I've seen plenty of scratched tint film and shattered glass to learn that lesson!
After installing or overseeing well over a square mile (literally) of Dynamat damping and sound control materials; I personally prefer to cut out the circle of damping sheet from the speaker cutout (rather than fold it in). Besides lowering the chance of reducing the opening size too much; I like to stick this circle (and any sizable scraps) on the outer door skin (on the inside of the door of course). We want to minimize resonances on the outer skin too!
Be sure to clean the surface thoroughly before applying adhesive damping materials; and always use a roller (I prefer wood) to maximize effectiveness.
On frequency response...
Musicians and their balance in the mix is the macro perspective. How their instruments actually sound is a micro perspective. Respect the art/science balance when evaluating.
It is better to cut an EQ/tone adjustment rather than boost. Consider what (frequencies/instruments) you have too much of rather than getting too fixated on what you don't have enough of.
Enjoy the ride.
