Purchasing new eGPU Setup for Mac
For some clarity for others, you can copy the exact configuration linked here without the need to add the resolutions you desire. It automatically adds a good selection.
For me, I did not realise that I could run the monitor at 3840x1620. This option is not available using a Macbook 13" 2018 with the Intel Iris Plus 655. This is most likely as the integrated graphics can barely run it at the resolution it defaults to, which is 3360x1417 (bizarre resolution!).
I feel slight buyers remorse not getting the Radeon IIV as I could have grabbed one for about $680 USD but couldn't justify the extra expense. I may change my mind over the next couple of days trying to get this thing to play games in Bootcamp!