"How do you pick art?" Art picks you. I would do that slowly, and when you see a piece of art or a photograph or a wall sculpture that really grabs you, pick it up and find its place on that wall. Art is personal and truly in the eye of the beholder.

I would just get some very large, over sized art in a modern style. If you're trying to get some function out that space, you could get a skinny console table. I obviously don't know what that is a hallway to (or from), but I like to put bookcases in hallways. People always say there isn't room, but there is. I like it when there are items in a long hallway that make you "meander" through it, it helps to break it up if there are actually things that you have to weave your way through. You can always put a bench in a hallway as well.

If you have a pair or trio of small pictures that work well with the mirror frame and table (or a small wall shelf or box (perhaps with a door or drawer for hiding a cell phone and/or keys and/or hair brush for below the one small picture), you could leave the mirror vertical and hang it eye level but off center horizontally with a grouping of smaller items beside it. You might get some poster board and cut it to the shapes of your mirror, etc., and then use removable tape to test how they'll work spaced in different ways.

Your hallway lighting def needs to be changed, it looks yellowy which contributes to the darkness. Semi flush LEDs or even those wafer (easy to install) ’recessed’ lights will for sure bring a brightness and life up there. A wallpaper (cant tell your style) at the top of the stairs with maybe a coordinating color for the rest of the hall. Maybe larger more colorful art would be a good idea. I feel like the hallways in our homes are not a priority when decorating but can be a good spot to try new things. But I think you should start with lighting in the 3500k range w a dimmer.