ETag header does not chandes if pagecache changed. Idea is make ETag as md5 of filecache time modified at pagecontroller.php.

protected function lastModified($id, $group)
{
$app = Factory::getApplication();
$filename = 'administrator/cache/' . $group. '/' .md5($app->get('secret')) . '-cache-' . $group . '-' . md5(md5(JPATH_CONFIGURATION) . '-' . $id . '-' . $app->getLanguage()->get('tag')). '.php';
if (file_exists($filename)) {
date_default_timezone_set('your server timezone...');
return date("F d Y H:i:s", filemtime($filename));
} else {
return 'file_not_exists';
}
}

public function get($id = false, $group = 'page')
{
    // If an id is not given, generate it from the request
    if (!$id)
    {
        $id = $this->_makeId();
    }
    $lastModified = md5($this->lastModified($id, $group));
    //$lastModifiedGzip = $lastModified.'-gzip';
    //var_dump($lastModifiedGzip);
    // If the etag matches the page id ... set a no change header and exit : utilize browser cache
    if (!headers_sent() && isset($_SERVER['HTTP_IF_NONE_MATCH']))
    {
        $etag = stripslashes($_SERVER['HTTP_IF_NONE_MATCH']);
        $etag = str_replace('"', '', $etag);
        //var_dump($etag);

        if ($etag == $lastModified)
        {
            $browserCache = $this->options['browsercache'] ?? false;

            if ($browserCache)
            {
                $this->_noChange();
           }
        }
    }

    // We got a cache hit... set the etag header and echo the page data
    $data = $this->cache->get($id, $group);

    $this->_locktest = (object) array('locked' => null, 'locklooped' => null);

    if ($data === false)
    {
        $this->_locktest = $this->cache->lock($id, $group);

        // If locklooped is true try to get the cached data again; it could exist now.
        if ($this->_locktest->locked === true && $this->_locktest->locklooped === true)
        {
            $data = $this->cache->get($id, $group);
        }
    }

    if ($data !== false)
    {
        /*if ($this->_locktest->locked === true)
        {*/
            $this->cache->unlock($id, $group);
        /*}*/

        $data = unserialize(trim($data));
        $data = Cache::getWorkarounds($data);

        $this->_setEtag($lastModified);

        return $data;
    }

    // Set ID and group placeholders
    $this->_id    = $id;
    $this->_group = $group;

    return false;
}