eGPU automatically as primary GPU for wayland/gnome
With this patch for clutter and the mentioned udev rule, gnome on wayland runs perfectly fine and all applications will use the eGPU automatically, if connected (including steam). If you can wait, I am sure there will be updated packages for 3.38 as it was merged in that branch also.
To disconnect the eGPU, you will have to poweroff and restart.
Despite that, no special scripts or services are needed.
Finally a glmark2 of 11302 for my AMD Radeon RX 5700 XT compared to 2631 for the internal Intel(R) UHD Graphics 620 (WHL GT2).
Thanks for drawing attention to this. Wayland is seeing a lot of linux development so it's good to have an egpu option.