The easiest solution is to use mySerial.readBytes() or mySerial.readBytesUntil():
The only problem with that solution is that, unlike your current code, it will block the execution of the rest of your code until the full communication has been received from the RFID module (or the function times out). The time to receive the data is a matter of milliseconds, but in some cases that can be an unacceptably long time to block the code. In this case, you would want to keep your loop() function running and save data to the global buffer variable when it's available.