How can I replace the hyphens in front of child terms with something else?
Looks like there is no easy way to override them, they are added by TermSelection::getReferenceableEntities in core/modules/taxonomy/src/Plugin/EntityReferenceSelection/TermSelection.php:73:
$options[$vocabulary-&gt;id()][$term-&gt;id()] = str_repeat('-', $term-&gt;depth) . Html::escape($this-&gt;entityRepository-&gt;getTranslationFromContext($term)-&gt;label());

If you decide to edit it, I suggest using a Composer patch system afterwards if you are using Composer.