Get posts of a custom post type from category from taxonomy?

Since you’re using a custom taxonomy, you can’t use the category argument. You can find some information about custom taxonomy retrieval in this comment in the developer documentation.

According to that, I think your $categoryArgs should be:

$categoryArgs = array(
    'orderby' => 'post_date',
    'order' => 'DESC',
    'post_type' => $type,
    'tax_query' => array(
        array(
            'taxonomy' => $tax,
            'field' => 'slug',
            'terms' => $myPostTax->slug,
        )
    ),
);