The problem with using any type of polishing head or polish with a Dremel on a large area is that it simply can 't give a uniform finish.
No matter what or how you do it, the smaller polishing head will leave the surface with a very uneven, blotchy look.
To polish a large area, the bigger the polishing wheel is the more even and uniform the finish.
Professional polishers use very large diameter HARD polishing wheels that they stack to form wheels as much as 6 inches wide.
As example, Colt used to make their own polishing wheels out of wood, covered with walrus hide. These were over 1 1/2 feet in diameter.
This allows keeping a flat surface like a slide perfectly flat, and gives a very uniform unvarying finish.
Try to polish a big area, especially a flat surface with a Dremel and it'll look terrible.
If you want to polish a large surface like a pistol, do it by hand with a metal polish and a cloth. It's slow and won't bring the surface to as brilliant a polish as a professional can, but it's look okay and won't ruin the surface.
As an experiment, get a flat piece of steel and try to polish a larger area with a Dremel. It'll look uneven and terrible.