Thanks!
As the files are coming from an external app we have control over, we just made the app chunk the files and save each chunk into a separate file field in the form. We're sort of good for our current specific use case (apart from ballooning db backup as we use WAL archival).
I'm also leaning towards presigned urls, creating those two issues was mostly about setting stage for the proposal. Here it goes: Proposal: better large attachments handling via presigned upload urls for S3