Wemos Mini & Pro Micro Serial connection
You don't need a logic level conversion between 5 V Micro Arduino and 3.3 V Wemos D1 mini. The 5 V ATmega 328p reads 3.3 V as HIGH and the esp8266 can tolerate 5 V logic level for tests. For long term connection you can add logic level conversion with a voltage divider to convert the 5 V of ATmega TX to 3.3 V of esp8266 RX.
RX/TX pins on Arduino Mini are Serial.
RX/TX pins on Arduino Micro are Serial1.
Serial is only USB on Micro.