I'm wondering why I can't I use "Mirror Modifiers" already now for the feather duplication since I could wait with the application of these modifiers until I've done the UV unwrapping properly.
"Couldn't" wasn't the right word to use. I think I could have mirrored at this point as a temporary measure. In the end I merge all the wing feathers together into one mesh (with stacked UVs based on individual UV layout) and then mirror.
Honestly it's all preference from what I can remember. Just my way of navigating through modeling, mirroring, UV'ing, merging, rigging. It's a lot to juggle in a workflow and I'm certain my way isn't the only way and maybe not even the best.