If you keep the mirror, turn it vertically; should be no wider than 75% of the piece under it. I don't care for mirrors in the dining room and prefer art. Ladder shelves are too collegiate for my taste as well. I think all the decor items you have could make good anchors for various seasonal centerpieces on your table, rather than stored in plain site on shelves. Just search under Stories in the box above for "centerpiece" to find lots of good inspiration for making those. Right now what you have is too skimpy and plain. Learn to be generous with color and greenery on the table, but keep things low enough to see over them and have conversation. You don't want to have to remove them when you entertain. I find your rug too small; chairs will fall off it when pulled out and sat in. It should be 36-42" larger than your table top on all sides. Time to go on the hunt for good art; not at the big box stores though.

I like your color palette a lot, but I think the room would look better with less decor on that back wall. I'd lose all the shelves. I don't even think I'd add art to any of the other walls.

Your rug does look a hair small, but not offensively so. I'd keep it.

I love that chandelier! Its scale calls for some larger decor pieces, though. I would recommend simplifying the decor on the wall where the mirror is. Currently, you have many small items along that wall. A longer credenza, a bigger mirror in proportion to a longer credenza, and perhaps art or hurricane-style candle lanterns on either side of the mirror would look great.

Your color palette and stunning mirror have more impact when the space is simpler.

With art instead of mirror: