Remove tinyMCE from admin and replace with textarea

No need to reinvent the wheel – put your editor support back and tweak the settings:

function wpse_199918_wp_editor_settings( $settings, $editor_id ) {
    if ( $editor_id === 'content' && get_current_screen()->post_type === 'custom_post_type' ) {
        $settings['tinymce']   = false;
        $settings['quicktags'] = false;
        $settings['media_buttons'] = false;
    }

    return $settings;
}

add_filter( 'wp_editor_settings', 'wpse_199918_wp_editor_settings', 10, 2 );