myself I love your mantle the way it is..just my opinion..as for the decor, it depends on the style, my guess is that you are going somewhat rustic with a tad contemporary..am i right? I think i would find 2 long narrow matching pictures or mirrors or pieces of art and center one on each side wall of the fireplace above the mantle, one thing that really looks great is to layer..so if you use mirors or if you use an outdoor scene ..say with a lot of grasses..you could layer your grasses in front of and below the art on the wall..to tie in the art to the mantle, and then add a few other outdoorsy things to the mantle..the photographs are too small for there, put them somewhere else..although lovely they are just not the right scale.. do you happen to have a rock collection? it would be pretty on the mantle with an outdoor picture of a stream going over rocks along a meadow? with the grasses.. or a few beautiful unusual rocks on the mantle with the grasses..or a hurricane lamp with washed river stones filling it..clear glass..to tie in the rocks of the fireplace with your art. tell a story about what you like, if you love horses a horse running through a meadow, if you love old mills, an old mill with a creek and some old wooden architechtural pieces.. a tall grouping might work in the center going down to a shorter one on each side..or a tall one on each side of each wall with shorter in the center.. maybe two long art or mirror pieces, a much taller grass in the corner center and the smaller grasses just over the edges of the art on each side.. there are number of ways to do this..but you love it..leave it white unless you hate the white..i don't..i think white and stones is very updated rustic

"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.

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.

As to colors: poppy floral art and rug - both have orangey- red? you could zing with 3rd orange [glass bowl type thing]. Or just keep the blues, brown sofa, etc., and have enough going. if this didn't work? can wear it... their display photo, see dark / grey chair with it. https://www.etsy.com/listing/940959810/100-luxurious-soft-silk-vibrant-green?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=peacock+blue&ref=sr_gallery-1-4&bes=1