Have you considered faux painting. It would add a look of texture to the walls. Take a look at www.paintedpiecesltd.com. Some great textured looking ideas. Love the color of your wall paint

I think a photo collage would look great in your hallway. You could do the stairwell, but I think it may be a bit small for a collage from what I can see. Here are a couple samples:

Here's a good resource. I do also think the hallway would work best for your gallery. I really like the idea of the white frames to keep with the lightness of your home.

https://www.houzz.com/magazine/how-to-design-a-family-photo-wall-stsetivw-vs~21387870

Agreed - I think a photo collage would look great in the hallway. Not so much in the staircase though. It seems like too small of a space. Why not try to select 2 or three of the photo collection and enlarge them for the stairway space, while keeping the same framing style. Another alternative would be to go completely different and select a sculptured piece to fill the stairway (in a similar colour to your frames). This article might help you out - I got my ideas from here. https://www.houzz.com/magazine/art-rises-to-staircase-challenge-stsetivw-vs~1772190