What is the most important data (or strava features) you want to keep?

I was actually looking into this myself, and a few parts of strava are not that hard. For me the overview of my rides (including avg speed and distance is important), I quickly implemented a webpage with javascript and leaflet (map viewer), and I could present that data pretty quickly using geojson. I had an offline program which converted gpx traces to geojson, but I'm pretty sure I'm able to read gpx in javascript directly. The harder part is generating the gpx trace (which I used komoot app for), as this involves matching your trace (gps positions) on a map instead of using the data as is.