For offline navigation on Linux, have you looked at osmin? It was pretty decent on a PinePhone.
How do you handle power-off? Does Raspberry Pi just shut down? My thoughts were to use Alpine or some RAM-based OS that would not corrupt SD card or the hard drive.
I have been messing around with building an in-car navigation from e-waste for a while now. Right now, I settled on an old smartphone with OsmAnd and wrote my own app to view the reverse camera.
(I haven't really used them a lot in the heat yet) Last enclosure was ASA, but AFAIK, black ABS is OK too because black pigment absorbs most of the light/UV, preventing plastic from degrading as fast