Yes, searching for '"google fi"' in the forum leads to thousands of messages containing "google" ... not terribly useful. Incidentally, I only found this somewhat recent thread by searching ... wait for it ... on Google for '"grapheneos" "google fi"'.
Anyway, I came here to offer a more recent report on Fi and GrapheneOS. My experience since 2025030800 is that Fi service no longer works on secondary user profiles. My setup was working well until this week: Fi (and sandboxed Play Services) in primary user profile, multiple secondary profiles. Previous to 2025030800, 5G networking with Fi service worked fine in the secondary user profiles. Now when I switch away from the primary user, my signal strength icon shows reduced signal, then after several seconds, I get the '!' no signal icon and all connectivity is dead without wifi.
Installing the Fi app in the secondary profile does not help - as a matter of fact, I could never get the Fi app to work for calls, texts, or data outside of the primary user profile.