I went through and sound proofed the back of my bronco a couple years ago. Still need to do the front.
I had a good paint foundation where as you have now put down undercoating.
https://www.foambymail.com/GR-_BLACK/gym-rubber-black.html
This stuff is pretty amazing, it is closed cell so it is water proof. I used contact cement from Menards, brushed it on both surfaces. Seems to be sticking well in all positions.
Not sure what extent you are going to since you already painted the topper.
I put down killmat, gym foam on all of the sides and roof. Then I used mass loaded vinyl on the floor and sidewalls on top of all of that.
Also put in new weather stripping.
I should have taken decibel readings. I had no idea it would be that quiet. I can't wait to finish the rest of the truck. From the first drive it was like dead silence from the back. Just doing that much felt like I was driving a brand new vehicle. Results were far better than expected. You could have normal conversations with passengers in the front and back without yelling or having to repeat things.
You are probably not looking for that extensive of a reduction but the gym foam is pretty cheap and I will attest that it is a good product.