Your problem is that the program gets stuck in the for loop.

• You need to also create direction variable so the program that knows which if statement to execute.
• You need to create some other logic that will increase the i variable without stopping the whole program.

The code:

//Initialize the i variable globaly:
int i = 0;
bool direction = 0;


//Your function:
void with_millis() {
  unsigned long currentTime = millis();

  if ((currentTime - previousTime >= eventInterval) && direction == true) {
    digitalWrite(MOTOR_IN1, HIGH);
    digitalWrite(MOTOR_IN2, LOW);
    i++;
    if (i <= 255) {
      ledcWrite(0, i);
      previousTime = currentTime;
    } elif (i > 255) {
      i = 0;
      direction = false;
    }
  }

  if ((currentTime - previousTime >= eventInterval) && direction == false) {
    digitalWrite(MOTOR_IN1, LOW);
    digitalWrite(MOTOR_IN2, HIGH);
    i++;
    if (i <= 255) {
      ledcWrite(0, i);
      previousTime = currentTime;
    } elif (i > 255) {
      i = 0;
      direction = true;
    }
  }
}