So i am struggling to understand why someone has gone to lengths to make this rule?? it's not in any standard that I am aware of, and it's easy to do on other devices so slightly puzzled.
You are welcome to complain to the upstream maintainers of ConnMan who authored/coded the connection manager whose features and capabilities we expose via the dbus agent in the Settings add-on. TL/DR; we didn't code anything or go to any lengths.
Unless you're planning to submit upstream patches to change worklows; the workaround will be to create a systemd service that runs before kodi.target and after network-online.target, that applies your preferred DNS server config to the active ethernet service using a connmanctl command. You'll find some useful prior art for something similar in other forum posts from users trying to change service (routing) order for VPN connections (also done with connmanctl commands).
NB: initial post approval applies to everyone and while your posts might not be visible to all users, they will be visible to mods/staff who are most likely to be the people replying. It is done solely for anti-spam reasons only, and is effective, and the algorithm that auto-manages it normally lifts restrictions quickly. If you're deeply offended by it or ragingly impatient, you're probably in the wrong forum.