As someone who isn't a GIS person but is trying to make a map-centric app (hiking/backpacking, with activity tracking and route planning), any suggestions for r

As someone who isn't a GIS person but is trying to make a map-centric app (hiking/backpacking, with activity tracking and route planning), any suggestions for resources? I just finished implementing enough of the WMTS spec interact with the one provider I'm using so far (UK ordinance survey). The only library I could find to help that works on Android was proj4j, which I'm using to handle converting points to different CRSs. I'm making incredibly rookie mistakes like just realizing I need to project vector data into the same crs as the raster base map. I want to be able to transparently support changing the base map to one with a different crs and lines and points recorded in the previous crs show up in the "right places", but I can't figure out how that's possible. I think I've seen other apps do it.