vtm VS osmscout-server

You are correct, Valhalla is used for routing. Maps are rendered on the ELEMNT, BOLT and ROAM with mapsforge, BOLT2 with vtm. To look at map files there is cruiser cruiser (mapsforge) and cruiser-gl (vtm). Create updated maps with wahooMapsCreator. There is also some documentation how to add POI's or make a custom theme.

for an example. It's perfectly smooth. The application is closed source but the underlying renderer (VTM) is LGPL3. The renderer, https://github.com/mapsforge/vtm, is basically an OpenGL alternative for the mapsforge offline maps format and maintained by the same guy (the original mapsforge renderer is arguably a bit slow, struggling to hide its slowness behind cached tile bitmaps).

Pure Maps with OSM Scout Server for offline mapping and geolocation

- build a minimal web page showing a Leaflet or a MapLibre widget, connected to a backend built using a compiled language like C++ or D, itself connecting to OSM Scout Server to provide the tiles. Or to OSM Scout Server directly if it is possible.

The last option is probably the most lightweight solution, provided you probably have a browser already running on your phone. I'm not saying this out of my ass by the way, I'm building an SMS app using Svelte for the user interface and D for the backend connecting to the modem and managing the SQLite database. It's way faster than chatty.

[1] https://openrepos.net/content/rinigus/pure-maps

[2] https://rinigus.github.io/osmscout-server/

There are on device or even in browser renderers that only need remote or locally stored vector data to render the map. This moves the burde of rendering to user devices that should be more than capable for this today (especially if you use a GPU renderer) and makes you server into just a dumb data pipe that should be able to scale much better & more cheaply.

One such renderer available via Qt/QML:

https://doc.qt.io/qt-5/location-plugin-mapboxgl.html

Alternatively you can have a local daemon running that does the tile rendering on your device from offline data packs. This way even "legacy" apps requiring simple bitmap tiles will work without (major) changes. One such project providing this (among other APIs) is OSM Scout Server:

https://github.com/rinigus/osmscout-server