I can tell you that with 100% certainty that your ‘16 Denali being 4WD is not the reason it doesn’t have ACC. There are 4WD models equipped with ACC. Yours just simply didn’t have that option added.
If it was possible to add ACC it would be a pretty big undertaking. I’m guessing you’d be looking at minimum removal of front bumper cover (possibly more), additional front end sensors, modification to front bumper sensor wiring harness, modifying the BCM, software reflash, and then of course re-assembly. A lot of this software and BCM work would have to be performed at a GMC dealership so you’re paying dealership labor rates.
I wouldn’t be surprised if it was at minimum $3K+ to accomplish that with GM OEM parts.
I think there’s too much of the ACC features and functionality dependent on the factory wiring harnesses, sensors, BCM, and software integration for there to be any “aftermarket” option. My guess is the only way to accomplish adding this option after the fact would be with GM OEM parts.