Someone should make a service like this except you actually upload the file directly to the other user rather than uploading it to a 3rd party (encrypted or not). Yes I get you would have to wait for the user on the other end to connect to you before the transfer starts but if you're uploading 100+GB of data you're going to be leaving that browser tab open for awhile anyways.
Yes, there are relays to connect the clients together and then the transfer is direct.
Several years ago when I was doing consulting I had lots of clients that blocked all the normal file sharing domains to prevent people from getting files into servers but magic wormhole always worked for me. I'd stash a wormhole-william (magic wormhole compatible Go application) executable in our installer deliverable and then I could update the software without IT's help in the future (I often had RDP access). The headaches saved by cutting red tape were worth the risk for me.