How can I set a separator for a taxonomy field?
You can do this in Twig.

Place this file in the templates folder of your theme (field-tags is the field name):

field--field-tags.html.twig

 &lt;div{{ attributes }}&gt;
    {% for item in items %}
      {% if not loop.last %}
        &lt;span{{ item.attributes }}&gt;{{ item.content }} - &lt;/span&gt;
      {% else %}
        &lt;span{{ item.attributes }}&gt;{{ item.content }}&lt;/span&gt;
      {% endif %}
    {% endfor %}
  &lt;/div&gt;


To add classes use {{ attributes.addClass() }} or {{ item.attributes.addClass() }}, see https://drupal.stackexchange.com/a/212307/47547