Agreed on avoiding google play services (iirc it's a semi-anon system?) I would say though, OSMAnd is terrible for me on an older device. Search basically doesn't work at all, I usually attempt to scroll the map till I find a place I recognze then drop a pin if I have to use it. Locks up often, navigation info disappears often, no alternate route user selection, an bad recalculation if at all when you miss an exit ramp. There are billion settings, so Id imagine some of these issues are fixable, but out of the box it's a terrible experience in comparison. The best option for local navigation i've used is HEREWeGo. It's online only, and also has issues with recalculations if you find yourself in rural areas often like me. Way better on battery though, navigation is first class, and search works similar to old GoogleMaps. You can actually search places like food and see open hours etc. Organic maps is fine for trails and stuff, but driving navigation is also nearly useless for similar reasons to OSMAnd.

Sandboxed Google Play is confusing because like you asked, why bother using a de-googled phone? Well, it's still degoogled when using sandboxed mode: Built into GrapheneOS, Sandboxed Google Play lets you run Google apps (like banking apps or Google Maps) while keeping your phone’s strong privacy protections. Instead of giving Google Play Services full access to your phone, GrapheneOS runs it inside a regular app just like any other app, but it's sandboxing means it's in a separate profile with no permissions and no other apps, AND you don't have to be logged into Google AT ALL. In fact, it's an anon sign-in that's automatic. It never even asks you. Google Play Services only gets minimal system access it needs (like checking device integrity for banks). It's a walled-in scope this way, so it cannot see what other apps are doing, or see any of your personal data.

Can’t you just sandbox Google Maps and avoid the headache entirely?