Extend the contact form of the Enfold theme to add dynamic fields

I need to integrate a form to allow users to select one or more services they are interested in. The constraints are: - The list of available elements must be specified via an array (no backend management is required). - Each element can have one or more subelements (with unlimited levels) and the parent element must have a customizable label to describe the link with the children (element A -> "because of" -> element A1, element B -> "subservice" -> element B1, element B -> "subservice" -> element B2, element B2 -> "in favor of" -> element B2.1). - Each element will be identified by an alphanumeric ID and may contain other fields (added later). - If an element contains subelements, then the user can not choose the parent element but must choose one of the children. - Each time the user chooses a service, this must be added in a summary below (without reloading the entire page). - By submitting the form, only the IDs of the selected services (not their names) must be sent. - Sending the form must be compatible with the native management of the Enfold theme, so that the mail sent contains the list of selected items. Graphically there are no constraints, but the style of the form must be consistent with the style of Enfold. I hypothesize the use of the REST API for asynchronous information exchange. This feature should be inserted in a file included by a child theme.