a pickup truck !
... not a bike ?
Actually not a bad choice for January in northwest Mexico .Any and all gravel roads will be open to consideration since the pickup is about the most common vehicle for all the ranch lands and backcountry village access. Definitely good ground clearance and able to carry spare gas and extra tires.Big plus is you will be indifferent to weather ,especially cold or snowy ( up to a limit of course) weather in the Sierras, and it can snow seriously there.
Should be no problem putting together a bunch of gravel routes and some pavement.
How about starting from Douglas, ,south to Moctezuma(paved) and then onward to Sahuaripa,,Rosario and Cd Obregon , then to Alamos and from there take the gravel road east to Chinipas and all the way to San Rafael and Creel .This has a gnarly ( had to use that adjective !) mountain road section but if a KLR and potatochip delivery cube vans can do it so can you.
Go to Durango the city and follow the gravel routes south from Mezquital to Jesus Maria and west out to the Pacific coast highway Mex 15 and if the 3 weeks are nearing an end head north back to Nogales
There are zillions of other interior ranch roads you can ride around ,connect various cities, just follow the lesser roads on the Guia Roji atlas and fill your 3 weeks.
Don't think you need to look for "technical " challenges, might get yourself stuck or broken down in the middle of" nowhere" , just wear the truck out .