> Does every service have hooks for possible extensions? What if a new service doesn't fit existing extension hooks?

No, only few services define service extensions.

It's more common for services to be configured solely via their configuration struct.

See the following for docs:

* https://guix.gnu.org/manual/en/html_node/Service-Composition...

* https://guix.gnu.org/manual/en/html_node/Service-Types-and-S...

This is less flexible-by-default than NixOS module, where any module can modify any other module. That is by design. The Guix developers see NixOS's approach as failing the principle-of-least-authority, where any arbitrary module (even those imported via flakes) can add a root SSH key.

I use NixOS, but it's an interesting tradeoff.