Upload multiple large DICOM files to Google Cloud Healthcare

I try to allow our users to upload multiple DICOM files from a React app to a Google Healthcare DICOM store.

DICOM instance files are sent programmatically and directly to the DICOM store using Google Healthcare POST endpoint.

It works well with fast connections but with slow connections (let's say ADSL), many uploaded files end up with 409 status returned by Google API. More specifically series of 409 are returned after a delay(because of Google HTTP/2 parallel processing I guess)

The total size of uploaded files vary from 100Mo to 500Mo(for 100 up to 500 files).

I'm aware that users should not try to upload that amount of huges files using an ADSL connection but I try to figure out why some files are uploaded with a 200 status, and some with a 409(in the latter case the XML response from Google has a 272 FailureReason).

Is there a way to send uploaded files one by one to avoid serial error responses? Or some good practice when sending multiple large files to Google Cloud? Or should I simply discourage users to upload many big files using a poor connection?

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