What significance does the bull/goat/mountain have in relation to the old Elves? It was always my (loose) understanding that AoS was sort of re-imagined realms of stereotypically over the top elements from each faction
This isn't quite right. AoS is a whole new world/system. Imagine it more like 10,000 years in the future. A great many things are inspired by or influenced by the Old World. Especially those parts that are controlled by the new gods, who lived during the Old World. So the cows are new. They're suppose to be the mountain spirits of this new kingdom, and these particular elves have bonded with a more elemental side of the world then the old High Elves.
These new Lumineth are probably the closest successor of the High Elves we'll get, but their a new successor with new stuff. Not a perfect copy paste of the old. Their lore even has them being tutored by Tyrion and Teclis as an attempt to rebuild their old people. But rebuilding still ends up with a new result.
I think the Fyreslayers are a great example of this. Imagine the Slayer cult passed down generation to generation for 10,000 years, until its evolved from a small cult to an entire faction of believers. Then at some point they actually walked and fought with Grimnir himself. You get something deeply inspired by the Old World, but still totally new.