Mirror 2 Macs UK to US

Hi Guys

I need help / suggestions for a solution to sharing files between our UK and US offices.

Heres what we have: The same setup in each office - Mac Mini with High Sierra and Serve 5.5 which servs files (mainly large photoshop files) to the workstations in the studio.

Each Mac Mini is connected via Thunderbolt to a RAID which has the 'work' volume. The UK Raid is backed up by Carbon Copy Cloner to another raid (which is working perfectly so no need to change)

The UK office has 500mbps up and down fibre internet connection (Wiline) The US office has 700mbps up and down fibre internet connection (Virgin Media) Internet speeds are consistent and stable on both sides when tested. We can achieve transfer speeds of 17 MB/s when using services such as WeTransfer. Our ISP have installed VPN on our routers.

So here is what we need...

We need each of the RAIDs to Mirror. So they have the latest version of each file constantly. So staff members can access the files on the 'local' RAID for speed. Once the files is saved it will sync back with the other location.

Speed of transfer is very important as some of our files can be <10GB.

We have tried Chronosync but found we can only achieve transfer speeds of around 1 - 2 MB/s. Similarly we can connect to each server with SMB and get the same speed when copying a file via finder.

A speedtest (.net) on the US server shows full speed, however if we do a speedtest from the US using a UK (London) server we get a dramatic drop in speed. and vice-versa

Can anyone shed any light on the issue. Where the route of the traffic goes / needs to go and where the drop in speed is occurring.

Thank you in advance.