Worth trying butler while you wait on the ticket. It's itch's own command line uploader and it takes a completely different path from the browser upload, so a failure that happens right at the end of the progress bar is often just absent there.
butler push yourbuildfolder irog/return-from-the-tavern:windows
The reason it suits your case specifically is that butler uploads incrementally. It diffs against your last build and sends only the changed bytes, so a small fix in one of fifteen languages costs seconds rather than a full 10 MB round trip every time. We push every build this way and haven't used the browser uploader in months.
On your own theory, for what it's worth: there's no published monthly upload limit, and a quota would almost certainly reject you before the transfer rather than after it reached 100 percent.