Export all activities as share picture

There isn't a specific function to do what you're asking so you'd have to recreate the feature yourself. Grabbing the activity data is easy, so the two other pieces you need are the map and photos.

For photos there's an undocumented endpoint to retrieve activity photos at /activity/{activity_id}/photos?size=<size> (I recommend just using 2048 as the size, which is the maximum value)

For the map, you can use the polyline and overlay it onto a map tile using a 3rd party mapping service. There are a lot of options out there. I'm not in front of my computer right now but there may also be a static map URL included in the same structure as the polyline data?