@Hugsy I've had a chance to check out your Wireshark dumps and I believe the Duet is overloaded by network requests. For a similar reason we don't recommend putting your Duet on the public internet. Here some ideas how you can reduce the number of simultaneous requests:
Use only one browser window (DWC instance) per client PC
Increease the AJAX request interval in Settings -> Machine
Upgrade your setup to SBC mode (it doesn't have to be a Pi although that's the easiest way)
We're increasing the number of maximum TCP connections in the underlying IP stack in the next firmware version, perhaps that will resolve the spurious disconnects as well.