Uploading a workout to Strava via mobile from a bluetooth-device

Hello my friends
I am building a device to record specific types of workouts (based on Arduino). It saves the workouts in a its own storage, in a .GPX file (GPS/workout data), and the user can view some stats of them on the device.

Ideally, I would like the user to be able to upload the workout to Strava. Strava allows you to easily upload .GPX files via their computer-website, but you cannot do it directly via their app (as far as I can tell).
I would like to find an easy (user-friendly) way to upload a workout from the device over bluetooth to the Strava app on a smartphone.

Has anyone some experience in getting workout data uploaded to Strava over Bluetooth? Maybe using a 3rd party app? Or any other knowledge about uploading to another workout app via bluetooth? How did you do it?

I doubt that this is possible unless STRAVE would implement this in their app. I cannot see any reason why they would do that. There is no widely used Bluetooth Mass Storage profile. So, any implementation would be non-standard and therefore would requires constant maintenance as APIs e.g., from the OS or some required stack change.

On their web page all they need is login and file upload (which is implemented inside all browsers) and GPX decode.

Look at how Garmin has implemented this. You use their app to download the activity from a watch over Bluetooth. They have a contract with STRAVA to upload new activities from Garmin to STRAVA automatically. Or you upload the FIT file using the STRAVA web interface.