Thanks for the reply!
As an update, I bought the Mac mini and currently have it connected to 3 x 4k displays.
Looking at the GPU usage graphs, I've been able to reach about 80% of the limit of the Integrated UHD 630 with SketchUp alone, and using maximum texture sizes.
The only thing that's maxed out the GPU so far has been the screensaver I use.
I can orbit large models easily, but there's a slight delay when I stop orbiting, and before the textures load. It doesn't orbit quickly with the textures still visible - it changes to solid colours.
For the moment, I'm going to keep with the integrated graphics as this computer seems almost silent, but if I decide to buy an eGPU, I'll update this thread to show the comparisons.
This is with the 2018 i7 Mac mini, 6 core, and 64GB Ram - running SketchUp and Layout 2020