Bathroom addition - pluming drainage problem
I did exactly what u describe. Take a circular saw with a diamond blade and cut into the concrete on the outside of the pipesize. Small increments at a time. Cut the complete depth of the blade. Than take a concrete hammer and get rid of the inside between ur two cuts. If u have to go deeper just continue with the hammer. Add the pipe and new metal reinforcement and concrete. The project was not much fun.  The stability is not a concern. My project was 5 years ago. Prep you concrete that the old binds with the new. There are plenty of YouTube videos for that. There is also a binding compount. I used the mesh type of reinforcement but I did not connect it to the old concrete.