There are several places on the "Gringo Trail" where you are likely to meet other riders heading South. (see below).
Do consider May is beginning of HOT season ... by July as you go South, seasonal rains begin. Heaviest rains are usually in August & September. Rain will follow you all the way to Colombia. Low regions can rain most. (Jungle)
Rather than hugging the Gulf Coast (HOT, traffic, ugly), I would go inland, ride down the Sierra mountain range ... all the way to Guatemala.
It will be cooler temps riding in the altitude of Sierra mountains.
Also, you will pass through are some of the most beautiful and historic towns in Mexico. Plenty of opportunity to explore very remote areas, roads in all directions.
You could start in Chihuahau area, visiting Copper Canyon. Or you could come South through Monterey then cut over to Matehuala, then go to Real de Catorce, a quaint little town near there. Good for a day visit and many dirt roads going to middle of nowhere. (Peyote center for Indigenous people)
Continue South through San Luis Potosi, then cut over to Zacatecas. Both interesting cities. Big, but pretty nice. From Zaca, continue South through Aquas Calientes, then Leon, Guanajuato, San Miguel de Allende. All historic cities.
Perhaps route around Mexico City(DF)? (over 20 million people, and all have cars!) Very tough traffic and corrupt Moto Policia. But great if you give it time and KNOW the City. (I spent six weeks there working on a movie ... I like Mexico City, but hard for travelers, not recommended)
Take small roads South to Oaxaca. From Oaxaca you can explore to the West or East. Will be HOT unless you can go UP in altitude.
Now it heats up, no escape. From Oaxaca area head South to San Cristobal De Las Casas, then onto Guatemala. In Guatemala, stay in the highlands, cooler. Coastal areas are HOT & WET in rainy season.
You may run into fellow riders anywhere along the way, but a few popular places where riders hang out:
Copper Canyon area (Creel or Batopilas) (Chihuahua state)
Guanajuato, San Miguel de Allende
Oaxaca, San Cristobal,
Guatemala: around Lago Atitlan area, Antigua.
good luck, have fun, learn Spanish!