There used to be a problem in the past between DTX and HR. But this is long gone. So yeah, you could also try it. I doubt you’ll see any damage in the voice quality.
Once again I repeat : when you enable DTX, you will probably see better KPI. But beware of the voice quality ! That can only be assessed with your ears or some VQ tools.
Regarding AMR codec tuning, here again, I can’t tell you how to tune them… Test and learn… (I never tried myself). As you understood, the robust AMR Codecs are not so useful because the drop will happen anyway. Therefore it is really difficult to imagine how changing the codec thresholds would impact the KPIs.