You say that you have" about two weeks to get to Oaxaca" but make no mention of how much time you might have available for the return to Colorado. Has that been figured out or is this meant to be a one way ride with the afterthought of riding back as being a chore you would rather avoid ? It is as if you are saying - we have two weeks , entertain us .
Shipping back from Oaxaca a number of bikes that run flawlessly seems a shocking ,silly and expensive concept when there are so many great roads in Mexico that can be put together into a complete loop. You are not in Baja this time .Save the shipping idea as an emergency measure for the USA if you run into a winter storm in the last stint .

Really , it is time to pore over a bigger fold out map of Mexico . Study up on which highways you could put together into a route that does not retreat the same way as you went in . The highways running south from the border tend to be major arteries in the flatter regions , fast travel .
For fun and " adventure" ( a personal judgement) look at any of the roads that run off those major routes.
Any highway that runs between he coast and the interior of the country is guaranteed to have a lot of curves and mountain scenery .

If two weeks-ish is all you can manage then better restrict yourself to the northern portion of Mexico no further down than the Bajio . Maybe ride down through Copper Canyon country ,the Mex 40 LIBRE from Durango to MAZATLÁN ( que aventura!) south along the Mex 15 then at Estacion Ruiz pick up the highway east into the mountains, some gravel road involved easily handled by your bikes to Jesus Maria and onward to Zacatecas , then south to Guanajuato, San Miguel ,Bernal , and then through the Sierra Gorda where you can find some more gravel adventures IF you desire.
You could cross over eastward by way of Mex 120 through Jalpan and Xilitla to Mex 85
or from Bernal east to Zimapan then Mex 85 north to Tamazunchale
You could visit the Sotano de las Golondrinas for a peek - but no bungee jumping into it .
There. More than 2 weeks worth of " adventure"i

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