I worked with a couple of the commercial alternatives to aspera and aspera as well and I used UDT, Tsunami, GridFTP, syncthing[1] as a poor mans alternative to Aspera.

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 ...

3 https://github.com/xtaci/kcptun