Caching tiles for a route

Hello,

currently I´m developing an app for smartphones and Garmin Watch. The app is my bachelor´s thesis so it´s non-profit academical.

I need to download tiles for a route in a bounding box of that route for offline use from zoom X to zoom Y. The routes aren´t generally that long, because it´s focused mainly on hiking, cycling, running etc.

Is it legal to use OSM map tile servers to get the data or would I breach their Tile Usage Policy, if so, what provider would suit my needs best possibly for free, or free for academic purposes?