P0139 O2 sensor circuit slow reponse (Bank 1/Sensor 2)

No, it’s not dangerous to drive with this problem. A repair shop should charge you about $110 for a Bosch 13461 rear oxygen sensor. Then, add 1 hour(?) labor charge.
The exhaust might be repaired by welding. The dealer would only replace expensive parts.
Say, “Hello independent shop!”