Laura I think your upstairs art over that sofa is fine as is. Lovely set-up. Go ahead and keep the clutter pillows if you love them but don't be offended if guests with bad backs (hi there) need to put them on the floor in order to be comfortable. I personally dislike them, but it is because I see what my back will spasm out into just by looking at them.

Yes, more like this blue? https://www.potterybarn.com/products/lyndon-upholstered-armchair/?catalogId=41&sku=1592244&cm_ven=PLA&cm_cat=Google&cm_pla=Furniture%20%3E%20Chairs%20%3E%20Fabric&cm_ite=1592244&gclid=EAIaIQobChMIz5OfsrKU4gIVHrbACh3WggRUEAQYBCABEgIf3PD_BwE What about lightening up your coffee table, too? Side tables..I like this one but there are literally hundreds of side tables to choose from...it depends on your personal taste...

Your wall paint is such a nice, deep color, and the mirrors are so interesting, I would most likely leave it plain. But it really depends on the entire room and balance you want to achieve, as well as the feel — calm and serene? More artsy and lively? Maximalist? Minimal? Will there be curtains? Amount of art on other walls? All factor in. Hard to say for sure and know context just seeing this wall. Elegant and serene: This is also a nice look: What feels best to you? ETA: If you’re really attached to that piece, I’d turn it: Ignore the art, just using to show scale and placement:

I think an abstract landscape/water piece, with some color, would look fabulous there … and play into your wonderful views. It would also bring warmth in, while keeping with the very modern style of the room. I’d look on Etsy, on gallery sites, Ebay, and/or visit local places to find a piece that speaks to you. This is a prominent spot — I wouldn’t buy a big piece of generic wall filler. Art here could be the soul of the room. https://magazine.artland.com/top-10-landscape-artists-in-modern-and-contemporary-art/ I do like the idea of three b/w prints you posted — it brings to mind water movement.