Have you considered the fact that the little white spots may be pitting, or pitting with product stuck in them (I'm assuming the polish you used was white?). How many miles are on the vehicle? Do you have PPF (Paint Film Protection such as clear bra or Expel) on the front? Have you tried using Erasure or a rinseless/no-rinse wash solution to wipe down the bumper cover and remove any of the polish residue or polishing oils?
I know you said Shelby GT, but you didn't say what year, how many miles, and what type of driving conditions. Any vehicle (especially black) will show pitting in the front bumper cover after accumulating miles on it.
One factor that comes up a lot is after someone takes the time to polish a panel, and removes years of swirls and scratches, it reveals things not visible before.
If it's solvent popping or pitting, there isn't much polishing will do.
Taking a second look at your bumper cover, it would appear that it has some amount of mileage or road rash.
Here is an example of some repair work I did on a black bumper cover with 140K miles on it:
Here is the same spot after a wet-sand with 3000, DA with 105 followed by 205. There is still pitting but not as much:
I used the squeegee method of Dr. Color Chip mainly to "fill" the pitting.
At the end of the day, the repair I mention was an enormous job and the bumper cover wasn't as intricate as your Mustang. It was very time consuming, but the alternative was to take it to a body shop. In the future, it not something I'll ever do again, and just let the body shop fill and respray.
Better pictures properly uploaded to the free AGO gallery and posted in the thread may help your cause.
https://www.autogeekonline.net/foru...1320-how-work-pictures-discussion-forums.html