The flashlight is already bound to LB; holding it for a second vs tapping one of the back buttons doesn’t really seem like a worthwhile use to, same for the map (which can be opened directly by long pressing “start”; or “select” if you haven’t reversed their mapping in Steam input to get the correct button prompts).
Honestly, I’ve yet to find a good use for the back buttons in Starfield; I’m considering mapping one of them to F5 for quick save, but that’s generally just a double tap of start and one tap of A, so it’s not inconvenient enough for me to have gone with the button mapping.