Make it possible to update Allowed Origins

OK here's what I did to make it work, and it indeed works now... but I feel like maybe I did too much?
If this was indeed necessary, I suggest we add the same kind of form that in S3 File System CORS...

<?php

namespace Drupal\my_module\Form;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\StringTranslation\StringTranslationTrait;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\flysystem\FlysystemFactory;
use League\Flysystem\AwsS3v3\AwsS3Adapter;
use Aws\S3\S3Client;

/**
 * Config settings for My Module S3 Cors.
 * 
 * This code is based S3 File System CORS s3fs_cors S3fsCorsAdminForm.php but adapted for Flysystem S3
 */
class MYMODULES3CorsForm extends ConfigFormBase {

  use StringTranslationTrait;

  /** @var FlysystemFactory */
  private $flysystemFactory;

  /**
   * MYMODULES3CorsForm constructor.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The config factory interface.
   * @param FlysystemFactory $flysystem_factory
   *   The Flysystem Factory
   */
  public function __construct(ConfigFactoryInterface $config_factory, FlysystemFactory $flysystem_factory) {
    parent::__construct($config_factory);
    $this->flysystemFactory = $flysystem_factory;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('flysystem_factory')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'my_module_s3_cors_form';
  }

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return ['my_module_s3_cors.settings'];
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    // You can debug this to verify the CORS config of your bucket
//    /** @var AwsS3Adapter $s3_adapter */
//    $s3_adapter = $this->flysystemFactory->getPlugin('s3')->getAdapter();
//    /** @var S3Client $s3_client */
//    $s3_client = $s3_adapter->getClient();
//
//    $bucket_cors = $s3_client->getBucketCors([
//      'Bucket' => $s3_adapter->getBucket()
//    ]);


    $config = $this->config('my_module_s3_cors.settings');

    $form['my_module_s3_cors_origin'] = [
      '#type' => 'textfield',
      '#title' => $this->t('CORS Origin'),
      '#description' => $this->t('Please enter the URL from which your users access this website, e.g. <i>www.example.com</i>.
      You may optionally specifiy up to one wildcard, e.g. <i>*.example.com</i>.<br>
      Upon submitting this form, if this field is filled, your S3 bucket will be configured to allow CORS
      requests from the specified origin. If the field is empty, your bucket\'s CORS config will be deleted.'),
      '#default_value' => !empty($config->get('my_module_s3_cors_origin')) ? $config->get('my_module_s3_cors_origin') : '',
    ];

    $form = parent::buildForm($form, $form_state);
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    try {
      /** @var AwsS3Adapter $s3_adapter */
      $s3_adapter = $this->flysystemFactory->getPlugin('s3')->getAdapter();
      /** @var S3Client $s3_client */
      $s3_client = $s3_adapter->getClient();

      $cors_origin = $form_state->getValue('my_module_s3_cors_origin');
      $this->config('my_module_s3_cors.settings')
        ->set('my_module_s3_cors_origin', $cors_origin)
        ->save();


      if (!empty($cors_origin)) {
        // Create an array of allowed CORS origins
        $cors_origins = array_filter(explode(',', str_replace(' ', ',', $cors_origin)));
        $allowed_origins = [];
        foreach ($cors_origins as $origin) {
          $allowed_origins[] = 'http://' . $origin;
          $allowed_origins[] = 'https://' . $origin;
        }
        $s3_client->putBucketCors([
          // REQUIRED.
          'Bucket' => $s3_adapter->getBucket(),
          // REQUIRED.
          'CORSConfiguration' => [
            // REQUIRED.
            'CORSRules' => [
              [
                'AllowedHeaders' => ['*'],
                'ExposeHeaders' => ['x-amz-version-id'],
                'AllowedMethods' => ['POST'],
                'MaxAgeSeconds' => 3000,
                'AllowedOrigins' => $allowed_origins,
              ],
              [
                'AllowedMethods' => ['GET'],
                'AllowedOrigins' => ['*'],
              ],
              // ...
            ],
          ],
        ]);
        $this->messenger()
          ->addMessage($this->t("CORS settings have been succesfully updated at AWS CORS"));
      }
      else {
        // If $form_state['values']['my_module_s3_cors_origin'] is empty, that means we
        // need to delete their bucket's CORS config.
        $s3_client->deleteBucketCors([
          'Bucket' => $s3_adapter->getBucket(),
        ]);
        $this->messenger()
          ->addMessage($this->t("CORS settings have been deleted succesfully"));
      }
    } catch (\Throwable $t) {
      $this->messenger()->addError($t->getMessage());
    }

  }

}
?>

No need for support indeed since I found a solution, but I suggested a form to update allowed origins like I did could be added to the module. Switching to feature request then.