To be clear, there is no need to replace the stock firmware unless you are taking a hard ideological stance. You can install F-Droid on most any Android phone, then install OSMAnd+ from F-Droid over WiFi, download some maps, and then turn off the networks to use it offline as a GPS-only device.
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...
GrapheneOS is available for the 4a.