I don't really like categories. I prefer tags. Let's hide what's not going to be used.

functions.php

<?php

namespace ChangeMe\Theme\Modules;

class Core {

  public function __construct() {
    add_action('admin_init', array($this, 'disable_category_feature'));
    add_filter('rewrite_rules_array', array($this, 'remove_category_rewrites'));
  }

  public function disable_category_feature() {
    unregister_widget('WP_Widget_Categories'); // Remove widget.
    register_taxonomy('category', array()); // Unhook all the category "functionality" but leave the data.
    remove_submenu_page('edit.php', 'edit-tags.php?taxonomy=category'); // Hide Administration menu.
  }

  // Permalinks for categories can go.
  public function remove_category_rewrites($rules) {
    foreach($rules as $rule => $rewrite) {
        if(preg_match('/^category/', $rule)) {
            unset($rules[$rule]);
        }
    }

    return $rules;
  }

}

$theme = new \ChangeMe\Theme\Modules\Core();