Not a direct answer but a pointer to a possible direction. Most dropzone type plugins (scan around as there are many eg The Almighty Dropzone) have a feature of emitting a Bubble event along the lines of “all files have completed uploading” and publish states of the files that succeeded and those that failed. The ones I know of are for uploading to external storage providers - but Bubble is not a lot different (other than cost!) to external storage providers.
This is the only way I know of getting an asynchronous event notification that files have completed uploading. Techniques like waiting a period of time, polling the DB etc are never going to reliably work.