If you are restricted to 5 or 6 days in Mexico I might suggest you race to the border at Eagle Pass that way avoiding a long swath of hot Texas- and getting into hot Mexico instead. But you will love it there.You will have to watch out for the occasional afternoon thunderstorms
especially if you get into western high country
This would be a better route for you would since you bypass the gulf lowlands and the desert west of Saltillo. (These are nicer to exlplore during cooler season, and you will shave a day or more off )
From Piedras Negras a possible route would go via Monclova, Cuatrocienegas ,Torreon and then to Hidalgo del Parral
There depending on how you feel about the available time you can short cut north to Mex 16 and to Cuauhtemoc and along the north of Copper Canyon country , through Basaseachic to Hermosillo ...
you could dive right into the thick of the CC via Balleza,Guachochi,Creel and Basaseachic
and then to Hermosillo
All roads I include here are first class paved highways, no dirt road stuff.
From Hermosillo it is a simple matter of " slabbing" north on Mex 15 to Nogales and Tucson
With only 5-6 days it can be done but it leaves not much time for dawdling and ejoying scenic stops too much.Remember you will NOT be riding on the open country highways after dark- too many horses ,donkeys,cows ,deer, javelinas and other animals to run into.And you don't see anything Mexican at night, just the spot of your headlight.If you can force yourself to get up real early and ride with sunrise you will have long days and can then afford to spend somewhat more time on scenery.