// https://wokwi.com/projects/327104226065908307
// for https://forum.arduino.cc/t/problems-with-interrupt-not-always-working-fast-enough/968966/34

const int myLED = LED_BUILTIN;
unsigned long blinkRateMs = 500;

void setup() {
  pinMode(myLED, OUTPUT);
  Serial.begin(115200);
}

void loop() {
  blinkLED();
  //:
  // other stuff
  //:
}

//...
void blinkLED() {
  // keep the important values for next time around.
  const unsigned long blinkTimeMs = 50; // on time
  const unsigned long pauseTimeMs = 200; 
  const unsigned long longPauseTimeMs = 800;
  static unsigned long lastBlinkMs;
  static bool toggle = false;
  static bool shortPause = true;
  // is it time to blink ?
  if (millis() - lastBlinkMs >= blinkRateMs) {
    digitalWrite(myLED, toggle = !toggle); // flip the toggle state
    if (!toggle) {
      if (shortPause) {
        shortPause = false;
        blinkRateMs = longPauseTimeMs;
      } else {
        shortPause = true;
        blinkRateMs = pauseTimeMs;
      }
    }else{
      blinkRateMs = blinkTimeMs;
    }
    lastBlinkMs = millis(); // ready for next blink
    //Serial.print(blinkRateMs); Serial.print(",");
  }
}