Earlier quoted context omitted.
Looking through the GrapheneOS source, the servers may not be Google servers but the system is still designed to phone home. As such, have they solved the problem or is this just another case of "Dont' trust them, trust us instead." Has anyone succeeded in running multiboot on "smartphone" hardware, i.e., where the user can boot into a choice of kernel/userland. One choice might be Android, another might be GrapheneO…
Did you actually find any examples of GrapheneOS phoning home? GrapheneOS doesn't rely on any third-parties I'm aware of. The only service provided is over-the-air security updates. It doesn't even come with an app store (although you can install F-Droid). For that reason, GrapheneOS alone fits all three categories you mentioned: It is Android, it is GrapheneOS, and it is fully controllable / doesn't ship bloatware.
Connectivity check / time servers
https://grapheneos.org/articles/grapheneos-servers#grapheneo...
Amongst others.