How to set up CiviCRM on separate Drupal from primary site?
You can certainly use the API from Drupal 8 to access your CiviCRM on Drupal 7, this is a set up I encounter fairly regularly.

There are a few resources explaining how this can be done conceptually:


https://civicrm.org/blog/jaapjansma/using-civicrm-form-processor-extension-to-handle-form-submissions-from-an-external
How to get CiviCRM Views to work in Drupal 8 with separate CiviCRM database
https://docs.civicrm.org/civiproxy/en/latest/