Android: Notification Shows in Delay And Stops Updating When App Closed for 30 Seconds (on OnePlus 8T)

Google has its clock app, which includes its stopwatch. I'm currently trying to create in my app a (count-up) timer, or you can call it a stopwatch, that will be able to run in the background, and when it runs in the background I want it to also show a notification, that displays the time it counts and a "Stop" button (all of this happens in google clock app (see here)). For the timer in my app, I'm using a Handler that posts a Runnable, which is posting itself. I'm writing my app in Java.

the code defining the 'timer' (Handler and Runnable):

Handler timerHandler = new Handler();
Runnable timerRunnable = new Runnable() {
    @Override
    public void run() {
        long millis = System.currentTimeMillis() - startTime;
        seconds = (millis / 1000) + PrefUtil.getTimerSecondsPassed();
        timerHandler.postDelayed(this, 500);
    }
};

my onPause function:

@Override
public void onPause() {
    super.onPause();

    if (timerState == TimerState.Running) {
        timerHandler.removeCallbacks(timerRunnable);
        //TODO: start background timer, show notification
    }

    PrefUtil.setTimerSecondsPassed(seconds);
    PrefUtil.setTimerState(timerState);
}

How can I implement the background service and the notification in my app?

Edit

I've managed to succeed in creating a foreground service that runs my timer, but I have two problems:

1. When I run the app after something like 5 minutes, the notification shows up in a 10-second delay.
2. the notification stops updating after around 30 seconds from the time it starts/resumes (The timer keeps running in the background, but the notification won't keep updating with the timer).

Here's my Services code:

public class TimerService extends Service {

    Long startTime = 0L, seconds = 0L;
    boolean notificationJustStarted = true;
    Handler timerHandler = new Handler();
    Runnable timerRunnable;
    NotificationCompat.Builder timerNotificationBuilder = new NotificationCompat.Builder(this, CHANNEL_ID);
    public static final String TIMER_BROADCAST_ID = "TimerBroadcast";
    Intent timerBroadcastIntent = new Intent(TIMER_BROADCAST_ID);

    @Override
    public void onCreate() {
        Log.d(TAG, "onCreate: started service");
        startForeground(1, new NotificationCompat.Builder(this, CHANNEL_ID).setSmallIcon(R.drawable.timer).setContentTitle("Goal In Progress").build());
        super.onCreate();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        String goalName = intent.getStringExtra(PublicMethods.getAppContext().getString(R.string.timer_notification_service_current_goal_extra_name));
        startTime = System.currentTimeMillis();
        notificationJustStarted = true;
        timerRunnable = new Runnable() {
            @Override
            public void run() {
                long millis = System.currentTimeMillis() - startTime;
                seconds = (millis / 1000) + PrefUtil.getTimerSecondsPassed();
                updateNotification(goalName, seconds);
                timerHandler.postDelayed(this, 500);
            }
        };
        timerHandler.postDelayed(timerRunnable, 0);

        return START_STICKY;
    }

    public void updateNotification(String goalName, Long seconds) {
        try {
            if (notificationJustStarted) {
                Intent notificationIntent = new Intent(this, MainActivity.class);
                PendingIntent pendingIntent = PendingIntent.getActivity(this,
                        0, notificationIntent, PendingIntent.FLAG_IMMUTABLE);
                timerNotificationBuilder.setContentTitle("Goal In Progress")
                        .setOngoing(true)
                        .setSmallIcon(R.drawable.timer)
                        .setContentIntent(pendingIntent)
                        .setOnlyAlertOnce(true)
                        .setOngoing(true)
                        .setPriority(NotificationCompat.PRIORITY_MAX);
                notificationJustStarted = false;
            }

            timerNotificationBuilder.setContentText(goalName + " is in progress\nthis session's length: " + seconds);

            startForeground(1, timerNotificationBuilder.build());
        } catch (Exception e) {
            Log.d(TAG, "updateNotification: Couldn't display a notification, due to:");
            e.printStackTrace();
        }
    }

    @Override
    public void onDestroy() {
        timerHandler.removeCallbacks(timerRunnable);
        PrefUtil.setTimerSecondsPassed(seconds);
        super.onDestroy();
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

And here is how I start it in my fragment:

private void startTimerService() {
        Intent serviceIntent = new Intent(getContext(), TimerService.class);
        serviceIntent.putExtra(getString(R.string.timer_notification_service_current_goal_extra_name), "*Current Goal Name Here*");
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            Objects.requireNonNull(getContext()).startForegroundService(serviceIntent);
        }
}

UPDATE

When I run the app on google pixel emulator, I don't face any of the issues listed

Solution would work much better, even with enabled battery restrictions, if you will replace recursive postDelayed with scheduleAtFixedRate in your TimerService inside onStartCommand function. Something like this:

        TimerTask timerTaskNotify = new TimerTask() {
            @Override
            public void run() {
                // add a second to the counter
                seconds++;

                //update the notification with that second
                updateNotification(goalName, seconds);

                //Print the seconds
                Log.d("timerCount", seconds + "");

                //Save the seconds passed to shared preferences
                PrefUtil.setTimerSecondsPassed(TimerService.this,seconds);
            }
        };
        Timer timerNotify = new Timer();
        timerNotify.scheduleAtFixedRate(timerTaskNotify, 0, 1000);

P.S. I can update your git repository if you will grant permission)