I was looking for a solution like External registration and selfservice suggested by @kerasit in order to have an external registration interface (optionally moderated) and self-service towards multiple Nextcloud instances (by different Nextcloud partners), i.e. a multitenant interface where the user submits the registration form where he specifies in which instance he wants to have an account. The request is (optionally) evaluated by an admin and routed towards the Nextcloud instance for the creation. @kerasit: the solution you suggested with PWM is multitenant?
Not sure if this helps, but Nextcloud does something similar on nextcloud.com/sign-up. Perhaps this will help as a starting point if you want to develop something of your own.
But I also think that in your case, something like Keycloak or Authentik makes more sense. You can then connect it to multiple Nextcloud installations using OpenID, OIDC, LDAP, etc.
Yes. That would be possible. In truth, then any registration portal of your choosing can be used. The only matter is how to lead the users there in a meaningfull way. The key point is that a user is created in the designated authentication…system (Nextcloud itself, LDAP or whatever else you use as IDP/user backend).