In the past I see that Tsunami, UDT, or GridFTP were suggested. Any results?
For real transfer with big (> 500TB) Aspera will deliver what it says over WAN (over the Atlantic).
If you have better connections and not so much data syncthing will probably work if you can have someone manually take care of all exceptions.[2]
If you know your data and you can build quite a lot of stuff yourself you can get almost the same speed as Aspera with UDT.
There is also a go implementation of UDTs used by kcptun[3] but I haven't tried that.
1 https://github.com/syncthing/syncthing
2 I don't intend to disrespect syncthing here, but when your dealing with TB scale 24/7 things are never really up all the time, the network is unreliable, the sender host filesystem is corrupt and the receiver host filesystem is full or broken or ...
But it looks like GridFTP and Globus are basically dead https://opensciencegrid.org/technology/policy/gridftp-gsi-mi...
Tsunami sounds good but maybe needs some updating? I haven’t looked closely...
It's not free for "managed" (Enterprise) applications, though, which is what the OP seems to be looking for, and I'm not sure it's the best choice for high-speed.
Something that I think is free is the CERN FTS [2], which can use a GridFTP back-end, so possibly you can roll your own high-speed big-data infrastructure that way.
And personally, I'd like to see more people getting a Globus subscription. It's cost-effective when compared to tools like Aspera, and helps fund the development of Globus software and features.