I had a look around for expected behavior during large http 1.x POST requests, it looks like the standards suggest that browsers should monitor for 413 responses and terminate the body transmission but they don't:

https://stackoverflow.com/questions/18367824/how-to-cancel-http-upload-from-data-events/18370751#18370751

Chrome and Firefox have issues 4+ years old on this, doesn't look likely that they will be addressed any time soon.

I think a two stage approach may be better for our scenario, too bad, this almost worked!