At the moment according to brevo-php/docs/Model/SendSmtpEmail.md at main · getbrevo/brevo-php · GitHub the htmlContent parameter is completely ignored if setting a TemplateId. Similarly the user interface does not seem to have any option in the params for variables for ‹ htmlContent ›, it only allows text coming from the Contact object.
HTML body of the message ( Mandatory if ‹ templateId › is not passed, ignored if ‹ templateId › is passed )
Comparably Mailchimp’s Mandrill templates support mc:edit="body" which gets replaced with the body send via an API call Créer des zones de contenu modifiable avec le langage de modèle de Mailchimp | Mailchimp
This essentially makes connecting from an external system that generates emails dynamically impossible to use Templates from Brevo. In my case its Drupal generating a vast number of different possible emails, yet cannot use Brevo templates as a wrapper or the body content generated by Drupal is lost.