Signal’s default, well-supported installations use Google services

Signal only uses FCM for notifications, with a fallback mechanism (WebSockets) being available in all builds of the app, as well as Google Maps for location sharing (which most people probably don’t use anyway).

so unless you’re an extremely atypical user, those services are present on most of your contacts’ devices

Google Play services being present on people’s devices has nothing to do with Signal including the library. They are present on almost every Android device, because Google pressures OEMs to include them and grant them system level privileges.

Let’s also remember that E2EE doesn’t protect the endpionts

Yeah, but that’s the case with EVERY messenger app, so I really don’t know what your point is here?