But the main overview is if it is possible to render a shortcode of a custom field from a service page CPT custom field which has been added to a testimonial post inside the testimonial CPT - This then gets displayed via a loop (bricks) in a template back onto the service page.
I think it is possible. But you need to use the code to render the shortcode in a WYSIWYG field value. And I'm not sure if the Bricks builder can help this case, you can contact their support for more information. Please read more on the documentation https://docs.metabox.io/fields/wysiwyg/#template-usage
It's the PHP code to output the field value so you need to add it to the template file on your theme. If you want to output it in a block, please try to use the custom block of Meta Box. Please read more here https://docs.metabox.io/extensions/mb-blocks/