It depends how much time you have. The route you've mapped is the quickest way through the country from San Luis Potosí. It's also the most boring, both in terms of the roads and the towns. (We cycled the part between Coatzacoalcos and Villahermosa, which was fast but dull. I think there's much more of the same on the route you've plotted.) Down near sea level, it will be really hot in July and August.
I would go something like: San Luis Potosí > Guanajuato > Querétaro (via Dolores Hidalgo) > Puebla > Tehuacán > Oaxaca. We went via Mexico City and loved it, but you can go around it if you don't like big cities. If you have plenty of time and like climbing, there's some fabulous riding (paved and unpaved) in the Sierra Madre Oriental mountains east and north-east of Querétaro, especially in the Sierra Gorda, but it's not very direct - we treated it almost like a side trip.
From Oaxaca, you could drop straight south to the beach towns on the Pacific coast, either to Puerto Escondido (which we avoided) or the area around Mazunte, then follow the coast heading east. Alternatively, you could follow the more direct route south-east from Oaxaca to Tehuantepec. Either way, the natural route from Oaxaca runs via San Cristóbal, from where you have lots of options.
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Will Fulford-Jones
http://www.contoursofacountry.com