From https://www.arduino.cc/en/Reference/softwareSerial:
Not all pins on the Leonardo and Micro support change interrupts, so only the following can be used for RX: 8, 9, 10, 11, 14 (MISO), 15 (SCK), 16 (MOSI).
This also applies to the Pro Micro. So this means you can't use pin 5 for RX, that's the cause of your problem.
Unlike other Arduino boards, pins 0 and 1 on the Pro Micro is Serial1, not Serial. That means you can use hardware serial on those pins without interfering with your use of Serial to communicate with the computer or upload sketches.