With time for the ride to/from Mexico you are left with something like 2 weeks for Mexico itself, a nice span.
Suggest you just ride the highways south in Baja with side trips on short gravel roads to various coastal points. Not much enthusiasm for riding solo and in the dirt in hot weather in case you spill and get yourself marooned. Heat is actually not a great concern along the west coast of Baja where cold ocean currents keep things cool. Then when you turn inland at El Rosario you can ride over a ridgecrest and crash into a wall of hot air ( but a dry heat - unless it happens to be a thunderstorm)From Topo south on the mainland it will be warm, chance of thundershowers every day from about noon on.Just because it is the rainy season there is no guarantee that it will rain on you every single day.
Either stay the coast route past Tepic and to Puerto Vallarta or head inland up Mex 40 to Durango, but be prepared for rain in afternoons again.Any highway from the coast to the interior is going to have lots of scenery, lots of curves. If the rain gets too heavy be ready to stop off early at any convenient hotel and start riding again at daylight next day .
Just enjoy the ride the sights and the smells , take it slow, and do a few hundred km per day. Don't make any overambitious distance plans, keep your return date in mind. When your time is half used up start the return leg with the same loose approach

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93