I plant my pumpkins on the top of a slanting hill. I actually caged in my pumpkins 4 years ago as I think if they were able to move down the hill, they’d break the vines. There is a rock wall between my plantings and the grass. I made my own caged area. I have lots of wildlife who seem to enjoy my pumpkins, especially as fall sets in. I have to enclose many in mesh bags to keep from the animals. One year I lost several to animals. They wait for them to get a certain size and eat. It’s late so I cannot take a picture right now.

. Ply away the trim. How "off" is off? The trim looks an awful lot like the base board I see left of the door? Skim and drywall are two separate issue. Take it off, it may only need skim coating. In any case, you don't want it. If it's really off? You still don't want that trim - do something very intentional appearing as above or below

Because I still don't understand the space below I will have to wait for more pictures to answer about wood slats. I'd need to see the colors of the floors, the doors... the rails. I will say that that ray of light on that wall seems intentional and needed in a potentially dark stairway. You would change that concept with light absorbing wood slats. Where's the architect?

Williams Sonoma currently has several William Morris prints available in fabric pieces of various sizes. Napkins, tablecloths, placemats, and so on. One or more printed fabric pieces could be framed to match or coordinate with the two frames you have already. Alternatively, there are narrow woven Morris tapestries of various qualities and prices available online. They need careful sturdy hanging to look good.