Create Custom REST API Endpoint and prepare with CPT thumbnail image

add_filter( 'rest_prepare_(post_type)', 'function_name', 10, 3 );
function function_name( $data, $post, $request ) {
	$_data = $data->data;
	$thumbnail_id = get_post_thumbnail_id( $post->ID );
	$thumbnail_full = wp_get_attachment_image_src( $thumbnail_id, 'full' );
	$thumbnail_thumb = wp_get_attachment_image_src( $thumbnail_id, 'thumbnail' );
	$thumbnail_medium = wp_get_attachment_image_src( $thumbnail_id, 'medium' );
	$thumbnail_large = wp_get_attachment_image_src( $thumbnail_id, 'large' );
	if( ! empty($thumbnail_id) ) {
		$_data['featured_image_thumbnail_url'] = array('full'=>$thumbnail_full[0], 'thumbnail' => $thumbnail_thumb[0], 'medium' => $thumbnail_medium[0], 'large' => $thumbnail_large[0]  );
	} else {
		$_data['featured_image_thumbnail_url'] = null;
	}
	$post_meta = get_fields( $post->ID );
	$_data['post_metas'] = $post_meta;
	$data->data = $_data;
	return $data;
}

add_action( 'rest_api_init', 'endpoint_function_name_here' );

function endpoint_function_name_here() {
	register_rest_route( '/wp/v2', '/(custom slug)/', array(
		'methods'	=>	'GET',
		'callback'	=> 'custom_callback_function'
		)
	);
}

function custom_callback_function() {
	$get_post_data = get_posts(array('post_type' => '(post_type)', 'posts_per_page' => -1));
	$post_titles = wp_list_pluck( $get_post_data, 'post_title' );
	return $post_titles;
}