I am looking for an offline map that can import a large number (>50k) of POIs and overlay them onto the map. It should provide at least some basic search functions, e.g. show POIs sorted by distance and filter POIs by type. The app should be able to import files (gpx or csv), nice would be to import an entire folder hierarchy in one run so I don't have to load every single POI file individually. The app has to work offline, I am ok uploading the data through a server, but given the large number of points, I strongly prefer if I could just copy the POI folders onto my device.
This sounds very basic, but I haven't found anything suitable that even reaches the level of e.g. Basecamp + GPS.
As an example of what kind of map I'd be happy with: Here Maps is nice as an offline map, but I haven't found a way to import anything. BackCountry Navigator cannot deal with POIs at all.