Hi all, just informing that I am releasing Social auth nextcloud | Drupal.org , a drupal module to signin using Nextcloud account that uses the social api drupal module. So now is easier than before to link drupal and nextcloud users.
Under the hood it uses the thephpleague/oauth2-client and bahuma20/oauth2-nextcloud libraries (which I helped writing tests).
Next step will be using nextcloud as Digital Asset Manager for drupal.