Overriding class AttachmentProcessorGD

Hello, I am trying to size fonts of watermark depending on image size, and since I want that plugin remains updatable, I am trying to override only apply_text_watermark method of AttachmentProcessorGD class. I put code in mu-plugin and class definition in a filter easy-watermark/available-processors. However, the method is not overriden. How to do it properly? Below is the code:

<?php
/*
  Plugin Name: modify easy watermark plugin
  Description: Modify text size to put on image watermark.
  Version: 1.0.0
  Author: 
  Author URI: 
  License: No license
*/

namespace EasyWatermark\AttachmentProcessor;

defined( 'ABSPATH' ) || exit;

return;

\add_filter( 'easy-watermark/available-processors', 'EasyWatermark\AttachmentProcessor\extendedProcessor' );

function extendedProcessor() {

		// \spl_autoload_register( function() {
			class AttachmentProcessorGDExt extends AttachmentProcessorGD {
				/**
					 * Applies text watermark
					 *
					 * @param  Watermark $watermark Watermark object.
					 * @return mixed
					 */
					private function apply_text_watermark( $watermark ) {

						echo 'rabotajet';
						return;

						$output_image = $this->get_output_image();

						if ( ! $output_image ) {
							return new WP_Error( 'gd_error', __( 'Could not create output image. Please check your server configuration.', 'easy-watermark' ) );
						}

						$font = Text::get_font_path( $watermark->font );

						if ( ! $font ) {
							/* translators: font name */
							return new WP_Error( 'font_not_found', sprintf( __( 'Font "%s" could not be found. ', 'easy-watermark' ), $watermark->font ) );
						}

						$text_size  = $this->calculate_text_size( $watermark->text_size, $watermark->text_angle, $font, $watermark->text );
						$image_size = $this->get_input_image_size();

						// correct iamge size

						$color_rgb = $this->get_rgb_color( $watermark->text_color );
						$color     = imagecolorallocatealpha(
							$output_image,
							$color_rgb['red'],
							$color_rgb['green'],
							$color_rgb['blue'],
							127 * ( 100 - $watermark->opacity ) / 100
						);

						if ( false === $color ) {
							return new WP_Error( 'gd_error', __( 'Something went wrong while allocating text color.', 'easy-watermark' ) );
						}

						$offset_x = $this->compute_offset( $this->get_position( $watermark->alignment, 'x' ), $watermark->offset['x'], $image_size['width'], $text_size['width'] );
						$offset_y = $this->compute_offset( $this->get_position( $watermark->alignment, 'y' ), $watermark->offset['y'], $image_size['height'], $text_size['height'] );

						$result = imagettftext(
							$output_image,
							$watermark->text_size,
							$watermark->text_angle,
							$offset_x - $text_size['delta_x'], $offset_y - $text_size['delta_y'],
							$color,
							$font,
							$watermark->text
						);

						if ( false === $result ) {
							return new WP_Error( 'gd_error', __( 'Something went wrong while applying watermark text.', 'easy-watermark' ) );
						}

						return true;

					}
				}
		// } );

	return [
		'gd' => [
			'label' => __( 'GD', 'easy-watermark' ),
			'class' => 'EasyWatermark\\AttachmentProcessor\\AttachmentProcessorGDExt',
		],
	];
}