I was at IKEA over the weekend. The Soderhamn chairs seemed massive, like they would overwhelm your space. I would check out West Elm’s options. They have a pretty big selection of chairs and in my experience aren’t so huge, but still cozy. They should also have a NYC location so you can sit in the chairs. Also, they have chairs in color, since you don’t want the room to be too gray. Use the opportunity to get a fun color like teal or pink.

I agree with Flo. NYC apartments are usually space challenged, and yours seems true to form. I’m trying to figure out the relationship between couch and desk and hall. Your art over the sofa should cover 2/3 - 3/4 the sofa width and hang 5 - 8 inches above. I’d do a square or portrait format canvas or print over the desk. If you want a bit of bling there, try Great Big Canvas’s metallic art: https://www.greatbigcanvas.com/category/art-by-color/metallic-wall-art/gold/?pagesize=90

A small chandelier would add a lot! Even in a rental you can replace a ceiling light, provided that it's installed correctly with a junction box, which is probable (except in my house, where about half the lights are installed without junction boxes - no code enforcement when it was built). Just save the original fixture to reinstall when you move. One that might be particularly nice is the Ramsele pendant lamp from Ikea, which is ingeniously self-dimming (pulling a cord makes it more open or more closed). Also ideal for renting: peel-and-stick removable wallpaper, perhaps for the headboard wall. Something subtle like a faux grasscloth is what I would aim for, not a pronounced pattern.

If you keep the stairs as is, you might consider using the entire space beneath them as storage space -- even the steps can be made into drawers facing the person walking up them and/or the storage can be accessible from the side. Consider the possibility of adding a shelf (with or without a hinge top door) around the room(s) above the windows for lesser used items (you'd need to have a (folding) steps tool or step ladder to access) -- books in open shelving in the living areas and, in the bedroom areas, winter items in summer and vise versa; holiday items; travel accessories, etc. BBQ tongs can help you retrieve a blanket from such shelves as fall turns to winter ... no step stool needed. Choosing vertical furnishings can give you more storage for less floor space used. A storage ottoman can do double duty as can some coffee tables. Using upscale wood filing cabinets for end tables and side tables can give you lockable storage.