Add the ability to redirect after password reset

This worked for me. It was not $form['#submit'] that I needed to alter, but instead $form['actions']['submit']['#submit'].

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_user_form_alter(&$form, FormStateInterface $form_state) {
  $form['actions']['submit']['#submit'][] = '_mymodule_homepage_redirect_submit_handler';
}

/**
 * Custom submit callback to redirect to the homepage.
 */
function _mymodule_homepage_redirect_submit_handler(array &$form, FormStateInterface $form_state) {
  // Redirect the user to the homepage.
  $form_state->setRedirect('<front>');
}