Hey @Armage
I guess I can help you more on that, at the moment the user_saml app only acts as a SP. This gives us more flexibility and also integrates with more than “just” Shibboleth.
However, there is a new feature request from this morning that is about adding support for an external service provider. I think that’s a feasible idea and will see if I find some time to add that for our next major release.
At the moment though, only using Nextcloud as service provider is supported.
Can you share your configuration with us? Shibboleth should actually work quite fine, see the following topic that contains some examples for Shibboleth:
Not yet. At the moment it is however supported to use it in combination with another user backend. So if you have user_ldap configured the whole group management and user information would be pulled from there.
Can you maybe open a feature request at Issues · nextcloud/user_saml · GitHub and describe what the advantages on this approach would be?
Thanks a lot! And if you have any more questions please let me know. If you have specific feature requests or encounter bugs please file them directly on GitHub. 
Cheers,
Lukas