It's not easy, is it? I think you have a FAB assortment of items for different situations.
I haven't had to think about formal events in so long I can barely remember what they are like. But I do have a few toppers for dressy events that I have used in different circumstances.
1. Vintage Biba jacquard kimono. Can be worn belted or loose. Pros: Perfect summer weight topper for a sheath dress. Cons: doesn't work with full skirts.
2. Textured tuxedo jacket. Pros -- easy to use to "dress up" less dressy items and also works with a sheath. Cons: does not work as well with a fuller skirt because it is not quite cropped enough. However, it can be used in a pinch.
3. Hilary Radley wool coat. It's outerwear and warm and soft and luxe feeling. But knee length so doesn't work with longer dresses.
4. Theory trench -- easy to throw on, works with many styles of dress because of its cut. But not warm for winter.
5. "Snow queen" fake Max Mara coat. Pros -- super dressy and fun and flattering, works with lots of styles of dress. Cons: outerwear but not that warm.
I would like to add a silver puffer jacket, short. And a longer (midi length) wool coat. But they are not immediate priorities because I simply don't have the occasions for wearing them enough as things stand.