"Should I move it right next to the shorter cabinet?" I wouldn't. It's hard to suggest a spot for it without seeing the entire room. I meant to say I like your arched firebox opening. Just wondering if a simpler arched screen might be a better fit for it. ETA - I know this is an entirely different style, so just overlook that. LOL, I wish our fireplaces looked like this! However, for me it still shows how just art and a couple other items can be simple yet make a mantel look nice. It doesn't take much to get more.

I agree with Judy G that one piece of art centered over the firebox and highlighted by the single light would be better. It looks like you have an interesting photograph to the right of your fireplace. It looks too small to go over the mantle, but perhaps something that works well with that. You don’t need to add items along the whole width of the mantle. It would be more effective to provide focus in the center. The narrow mantle does look a bit stark against the stone in the photos, and might be improved by being closer to the stone color, again as Judy G proposed. Two other thoughts - to balance the hearth better, perhaps move the two tall candlesticks to the right side of the hearth, or find something else you like to balance the firewood holder. Finally your furniture arrangement (which I realize is not part of what you asked for advice on) seems misaligned. It is a long room. Perhaps either shift the sofa toward the fireplace, even though it wouldn’t be centered on the wall, or center these two chairs on the sofa, and find a wonderful, special, cozy chair to put to the right of the fireplace. Right now, it would be hard to have a conversation between the chairs and the sofa.

I think your current mantle is too thin, short and dark. I replaced it w/something like this I put a throw in the basket for some more texture. take away the other flower pot next to the tools. in the season where you aren't using the tools, remove it, replace w/something like this. this is one of my favorites, but only if you have something interesting to reflect I like this too. this mantle is similar to yours but it goes all the way across. try copying what they've done here, only not as much (yours is a smaller length) gave you logs in the basket. btw, have you thought about doing a charcoal color on the walls (the top two here match great w/your mortar on the FP) they are grays that have yellow undertones, so they'd work perfectly w/the stones.

The mirror is perfectly placed. In my home I deliberately placed 2 mirrors near windows to capture the light.. The solution is to hang some art where the mirror reflects it. From other positions in the room, the mirror may reflect the outdoors. Hard to tell from the photo. Placing a mirror in the correct place in a room allows you to optimise natural light. To do so, place the mirror adjacent to a window to perfectly catch the angle of light and bounce it throughout the room. https://www.countryliving.com/uk/homes-interiors/interiors/how-to/a854/how-to-use-mirrors-to-make-home-bigger-brighter/ Hanging a mirror in a dark part of a room does nothing to increase the brightness in a room. A mirror can only reflect the light that hits it, so it makes sense to put it where the light hits the wall, near a window.