Given the ESP32 capabilities the library for serial connections are build around HardwareSerial. So is ‘Serial’ defined as HardwareSerial(0), Serial1 as HardwareSerial(1). The RX/TX pins for serial connection can be assigned to nearly any pin.
I had look at the library to see whether it would be as easy as define DMXSerial as Serial2 but it requires more changes on a couple of places: