Entity Form: Override Submit Handler with Custom One

Question

I'm loading a field collection item form inside of a formatter. Below is the function loading the form, which works. However I do not want to call the normal field collection submitForm handler, I want to override the submit handler with my own. I've tried attaching my submit handler, but the field collection submit handler still continues getting called.

I don't want to use a form_alter, I'm trying to keep this in the FormBase or ContentEntityForm class. Does anyone know how I can override the field collection submit handler?

<?php
public function getFCForm(FieldItemInterface $item, $fc_name) {
    $entity = $item->getFieldCollectionItem();
    $displays = EntityViewDisplay::collectRenderDisplays([$entity], 'full');
    $display = $displays[$entity->bundle()];
    foreach ($display->getComponents() as $name => $options) {
      $fields_by_weight[$options['weight']] = $name;
    }
    ksort($fields_by_weight);

    //load fc entity
    $fieldCollection = \Drupal\field_collection\Entity\FieldCollection::load($fc_name);


    $field_collection_item = \Drupal::entityTypeManager()
      ->getStorage('field_collection_item')
      ->create([
        'field_name' => $fieldCollection->id(),
        'host_type' => 'node',
        'revision_id' => 0,
      ]);

    $form = \Drupal::service('entity.form_builder')->getForm($field_collection_item);
    $form['#prefix'] = '<div id="inline_fc_form">';
    $form['#suffix'] = '</div>';
    $form['actions']['submit']['#submit']=['::ajaxFormSubmitHandler'];
    $form['actions']['submit']['#ajax_processed'] = TRUE;
    $form['actions']['submit']['#value'] = t('wtf');
    $form['#validate'] = ['::validateForm'];
    $form['#submit'] = ['::ajaxFormSubmitHandler'];

    //custom submit handler not being called?

    return $form;
  }