How to assign Drupal's system roles to Case Role in CiviCase
Welcome to SE!  You're right that Case roles are not the same as Drupal Roles.

Case Roles are relationships. There is a module called "CiviCRM Relationship to Role Sync" that might help which is part of the civicrm_entity module, but not enabled by default.  Someone was asking about this just yesterday on mattermost and it seems we don't have much current documentation for it.

Another approach would be to use Drupal Roles and a post by @petednz might help: https://fuzion.co.nz/blogs/complex-civicrm-relationship-role-synchronisation-recipe