Fading Neopixels consecutively on

I have the following code which is used to consecutively turn neopixels on in my strip. Problem is I would like to try and have it so the pixels slowly fade in.

Can anyone recommend what is the best way to try and do this.

Thank you

#include <Adafruit_NeoPixel.h>

#define PIN 8
#define NUM_LED 6

Adafruit_NeoPixel strip = Adafruit_NeoPixel(NUM_LED, PIN, NEO_GRB + NEO_KHZ800);
const uint32_t RED = strip.Color(255, 0, 0);
const uint32_t OFF = strip.Color(0, 0, 0);
unsigned long startTime = millis();
boolean isFiveSecondsElapsed = false; 
uint8_t index = 0;
  
void setup() {
  strip.begin();
  strip.setBrightness(64);
  SetAllPixelsToOff();
  strip.show(); // Initialize all pixels to 'off'
  
}

void loop() {
  if (millis() - startTime >= 5000)
  {
    isFiveSecondsElapsed = true;
  }
  
  if (isFiveSecondsElapsed)
  {
    strip.setPixelColor(index, RED);
    strip.show();
    
    index++;
    if (index >6) 
    {
      index = 0;
      SetAllPixelsToOff();
    } 
    
    isFiveSecondsElapsed = false;
    startTime = millis();
  }
}

void SetAllPixelsToOff()
{
  for (uint8_t i = 0; i < NUM_LED; i++)
  {
    strip.setPixelColor(i, OFF);
  }
  strip.show();
}

What is the point of using a boolean isFiveSecondsElapsed? It’s completely redundant. Instead, just:

if (millis() - startTime >= 5000)
  {
     strip.setPixelColor(index, RED);
    strip.show();   
    index++;
    if (index >6)
    {
      index = 0;
      SetAllPixelsToOff();
    }   
    startTime = millis();
  }

Okay thanks. (Still learning)

Do you have any idea how to go about the fade on?