Your Roomba should be 5V tollerant so no voltage divider needed to send data to the Roomba.
Your ESP however does need a voltage divider on it’s RX (if used)
But the code only sends data to the Roomba so #define SERIAL_RX D5 // pin for SoftwareSerial RX isn’t needed.
I was planning to get readings from the Roomba but haven’t had the time to play with it yet.
Not sure how the esp-12E is different from the nodemcu, I think D6 should work.
Bare in mind that my roomba is rather new, I think the older ones might run on a different baud.