There are some things that you can do. They are a big metal box, and so some booming is to be expected, however you can cut it down quite a bit if you want.
Home Depot and other hardware store carry Flashing tape. It is almost as good as the expensive sound deadener that you can buy, but way cheaper. Put this on the roof under the headliner, in the doors, under the carpet, rear quarter panels, anywhere there is large expanses of sheet metal. It will deaden it and cut down a lot on the booming.
https://www.homedepot.com/p/Nashua-T...3003/206495170
The other thing is to keep doing what you are doing, reduce vibrations. Balance the drive shafts, maybe use balance beads in the tires or have them balanced. Exhaust can cause booming also, make sure it is in good shape and exiting out the back corner, not straight back. Exhaust fumes can get sucked in around faulty rear window/door seals.
Shocks and suspension bushings can also add to the noise in the cabin, make sure they are all in good shape.
I would highly recommend replacing all the door striker bolts. From the factory they had a plastic bushing on them, over time this bushing goes bad and falls off. Now the doors rattle and add to the booming. I just did all 4 on my 90 Sub and it made a huge difference. Then adjust the hinges so the door closes correctly and does not bang or pop up when the door hits the striker. I also tightened up the rear doors (barn doors) and that helped too.
I just drove my Sub this morning and it was nice and quiet. They will never be as quiet as a 2019 Suburban, but you can get them fairly close with some minor work.