I wonder if a console table with artwork(a diff piece) above it would give you what you're looking for. You can then add artwork to the other side of the door as well.

2 pieces side by side in that area or one very large piece (of framed art) would be my vote. Nothing on the long stair side wall as the windows give some interest and more framed art there would seem cluttered. Nice space!

I would install a series of glass, ceramic or metal shapes on the wall in an artistic fashion. Check out Wall Play from Gold Leaf.

I would do a big piece of art that fits the wall space and not worry about what you can see from elsewhere.

Find one large piece of art that you are in love with and you won't worry so much about the spacing. Do you really love this piece of art with the birds? It seems really washed out and underwhelming to me at the top of the stairs. If you love it, I'd find another place for it. Or, maybe it could just benefit from a slender metal frame around it to define it more?

The artist has also helped people find the perfect piece or grouping and it was not crazy expensive.

one large piece you LOVE. You always have to love it then it works anywhere IMO. It will never disappear or fight with the view it just is, I sugget you go to an art college to see if there is something that catches your eye , you get to meet the artist and maybe someday a pice done on commission just for you. We actually made a pact along time ago that we always want to meet the artist before buying it makes it a bit more interesting for us.