Nice analysis — the Europe concentration you found matches what I see live: I run a map of SatNOGS receptions for the ~23 open-telemetry cubesats (https://overwatch.confinia.io, discussed in the control-room thread), and the station density gradient is just as striking in real time.
If it helps a next iteration: the site exposes free API endpoints with per-station reception aggregates over the tracked fleet — GET /api/v1/stations (frames, satellites heard, last reception per station, 7-day window) and /api/v1/stations/{callsign} for one station’s log. Might save the raw log crunching for this kind of study. Same CC-BY-SA data, attributed the same way you did.