wp_dropdown_categories() show elements checkbox

Good for @Tom J Nowell♦ solution would be more properly. But it still could fulfill the goal which add the extra function in your class.

class List_Categories_Radiobuttons extends Walker_Category
{
  function start_el(&$output, $category, $depth = 0, $args = array(), $current_object_id = 0)
  {
    $category_name = esc_attr($category->name);
    $radiobutton = '<input class="form-check-input" type="checkbox" value="' . $category_name . '" id="flexCheckDefault' . $category_name . '"><label class="form-check-label" for="flexCheckDefault' . $category_name . '">' . $category_name . '</label>';
    $output .= '<span class="form-check">' . $this->content_el() . '</span>';
  }

  function end_el(&$output, $category, $depth = 0, $args = array(), $current_object_id = 0)
  {
    $output .= '';
  }

  private function content_el()
  {
    $args = array(
      'taxonomy' => 'pa_rozmiar',
      'name' => 'pa_rozmiar',
      'value_field' => 'slug',
      'class' => 'd-none form-check',
      'walker' => new List_Categories_Radiobuttons
    );
    wp_dropdown_categories($args);
  }
}

The arguments is fixed so far.You could setup the argument and call the function when turn public. Then generate your $output later when you call end_el.There is more flexible is use DOMDocument to add your document node, and it would be manipulate like JavaScript(or JQuery) and easily.