Magento 2: How to sort order of custom links nav?

A little bit code modification is need in your code.

Open your di.xml then put sortOrder="-1"

Using this sortOrder="[NumericValue]" tag you can arrange sequesce of execution of plugin for the same method.

plugin sortOrder: The order in which plugins that call the same method are run. see http://devdocs.magento.com/guides/v2.0/extension-dev-guide/plugins.html

Reason:

Magento adds categories in the menu using a plugin of same class at Magento_Catalog.

 <type name="Magento\Theme\Block\Html\Topmenu">
     <plugin name="catalogTopmenu" type="Magento\Catalog\Plugin\Block\Topmenu" /> 
 </type>

So should run your plugin before that default plugin catalogTopmenu, we know that sequence of plugins can manage from sortOrder field,

That you need put sortOrder of your plugin in negative than catalogTopmenu.

Also if you don't put sort order in plugin then it means sortOrer value should be 0.

That reason, I'm suggesting to put sortOrder value -1.