Not sure what “airbrush” look people are talking about, other than noise reduction at work.
Having seen the XR in action, I actually found the selfies to be a lot better than my 6S. The smart HDR definitely helps a lot in getting better colors.
As for the rear facing camera, I find the XR to be excellent compared to my 6S. Again, the smart HDR can capture shadow highlight details that are completely missing on my 6S.
Sounds to me that you don’t like the smart HDR feature/noise reduction algorithm if the A12’s ISP. Unless Apple tune the algorithm differently in future OS updates, just go back to the 8+. The smart HDR is a new feature of the A12. Apple don’t usually change things on the older phones despite anecdotal comments. A lot of the features in the photography of iPhone relies on the ISP hardware.