Bus travel from Zihutanejo to SAN Miguel.

Your best bet is Primera Plus or Parhikuni and go to either Moreila or Queretaro and then get a different bus from either Morelia or Queretaro to SMA.
I think there is a Primera Plus that goes from 02:30 PM — 09:30 PM.

Also when you get to Qro. you could chose to take a cab directly to San Miguel. Should not be that expensive but busses run all the time as well.

I would not think there would be much issue with safety unless something specific arises and they stop running busses. But you would know that ahead of time.