Hi there,
Unfortunately the Breadcrumbs element in the Header is not a hook based feature. It is an element defined in the Header builder system. You can see the file which is responsible for the definition here:
wp-content/themes/pro/cornerstone/includes/elements/definitions/breadcrumbs.php
And the backend implementation here:
wp-content/themes/pro/cornerstone/includes/elements/mixins/breadcrumbs.php
And the frontend implementation:
wp-content/themes/pro/cornerstone/includes/views/elements/breadcrumbs.php
So you can not customize the Breadcrumbs element using the conventional WordPress Hook based system.
At the moment we do not have an API implemented to automate the process like the Classic elements. This feature is planned to be implemented in the late future.
Thank you for your understanding.