Hey,
OsmAnd sounds like the most proper mobile app for what you may want, although I don’t fully understand what you want. You want to route in areas where there’s little to no OSM coverage regarding roads/paths, and you want to take photos of the places those exist so to map them later?
OsmAnd has a plugin that allows you to take pictures/videos/audio notes with GPS on, so to help you later find easily where you were so to map it.