Earlier quoted context omitted.
... This is a _fascinating_ idea. I have a Pixel 4a right now, can I install this stuff on there and just switch to a dumb phone entirely for... phone purposes? It looks [0] like this shouldn't be too tough... [0] https://www.youtube.com/watch?v=PgNWaORMM9I
>I have a Pixel 4a right now, can I install this stuff on there GrapheneOS is available for the 4a.
Replacing the firmware with something de-googled would make it more certain that don't login with a Google account during initial phone setup nor send any telemetry during that period before you go offline, or if you go back online eventually for map updates etc.
Also, if you're going to use it offline as just a "GPS tool", you might not care about other issues like getting firmware updates. It won't be subject to attack if you're offline and not browsing the internet...