How to update entity value programatically after submitting the form?

Question

I try to update automatically entity title after form is sent. I tried that in form_alter, but everybody guided me to use entity presave. I cannot manage to save the updated title however (please see comments below in the code).

function discount_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  if (!empty($entity->toArray()['product_id'][0]['target_id'])) {
    $title = $entity->toArray()['title'][0]['value'];
    $product_id = (int)$entity->toArray()['product_id'][0]['target_id'];
    $product = \Drupal::entityTypeManager()->getStorage('commerce_product')->load($product_id);
    $events_dates = $product->get('event_date')->getValue();
    $count = count($events_dates);
    if (empty($events_dates[$count-1])){
      $count = $count - 1;
    }
    if ($count > 0) {
      $start_date = date("j/n/y", strtotime($events_dates[0]['value']));
      $end_date = date("j/n/y", strtotime($events_dates[$count-1]['end_value']));
      if ($count == 1) {
        $count_suffix = "zjazd";
      } else if ($count == 2 || $count == 3) {
        $count_suffix = "zjazdy";
      } else {
        $count_suffix = "zjazdów";
      }
      $date = $start_date.' - '.$end_date.' ('.$count.' '.$count_suffix.')';
    }

    /* if I kint($title.$date) here it gives me a proper value, so only the code below doesn't work */

    /* this code below doesn't update the title regardless if there is $entity->save() later on or not */
    /* I tried these 3 options below, none of them updates the title */
    $entity->title->value = $title.$date;
    $entity->setTitle($title.$date);
    $entity->set('title', $title.$date);
    
    /* When I do kint($entity->toArray()); here I can see the updated title, however it doesn't save it later do database */

    /* this line below causes endless loop, when it is commented it also doesn't save the title */
    $entity->save();
  }
}