Dichosalocura, I truly admire your willingness to make this trip by bus! Grupo Senda is certainly one option. The Grupo Senda bus ride from Guadalajara to San Antonio is 20 hours. The bus seats 42 people--a very tight squeeze, sitting bolt upright for that long, long trip. Cost per person is about $2300 pesos. Makes my eyes roll back in my head just to think about it--but it might be just the experience you seek. You're a stalwart person.
On the other hand, ETN Turistar offers service from GDL to Nuevo Laredo, México, where you would go across the border (taxi) to Laredo, Texas and catch a bus to San Antonio. The ETN Turistar buses have 24 seats that recline far enough that you can actually sleep more or less comfortably, they have lower leg/foot rests so you can really put your feet up as you ride, and the cost per person is $1390 pesos. The trip on ETN Turistar takes 13 hours to Nuevo Laredo and another 5.5 to San Antonio via Greyhound. The leg from Nuevo Laredo to San Antonio will cost about $30 USD per person.
I would far rather take ETN Turistar for less money, more space, and less time than ride for more money, more time, and FAR less space/comfort on Grupo Senda.
I use ETN all the time to go back and forth from Morelia to Mexico City and while that trip is only 4 hours, I've never been uncomfortable--far from it.
If you have INAPAM cards, either of those bus lines will discount your travel cost by 50%.
I didn't answer your question, but...well, it will be an interesting trip! More power to you, dichosa.