Is it possible to use Nextcloud Talk with clients/patients without having them as a user?
Yes, you can create conversations that are shared as a link and anyone with the knowledge of that link can join. Optionally you can also set a password.
See:
https://docs.nextcloud.com/server/latest/user_manual/ar/talk/talk_basics.html#managing-a-conversation
https://docs.nextcloud.com/server/latest/user_manual/ar/talk/join_a_call_or_chat_as_guest.html