bookreader That's the point though, having the phone software somehow bound to a particular service instead of allowing the user of the device to bind it to their OWN service.
For several services GrapheneOS provides the ability to choose between servers (often Google's vs. a GrapheneOS proxy), and in some cases AOSP allows configuring arbitrary servers.
It is true that the system updater and the Apps app are pointed at GrapheneOS servers, but pointing a GrapheneOS device at different update servers would require somebody to maintain those servers. If one wishes to do that, the GrapheneOS web site provides build directions for system images (including, if I recall, delta updates) and guidance on how to set up the update servers.
Overall I guess I am not yet understanding which enhancement(s) are wished for.