Nah, he should wait for a dem win in 2020 and get a cabinet appointment.
Cabinet Appointment doesn't seem to get texans very far... ie Castro.
HHS isn’t a big deal relatively speaking and if I recall, mayor of San Antonio is mostly a figurehead position. Joaquin is more of a heavyweight than Julian, I think.
John Delaney is a three term representative. I think Beto would have a shot. He has name recognition that Delaney doesn’t have. Assuming Beto loses in 2018, I think it would be silly for him not to try for 2020.