As a home stager and amazon affiliate, I still love the look of floating shelves: https://amzn.to/35yl8GU I also like the look of metal wall art and I'm not sure what your taste is. At Home and Amazon both have great metal wall art options. A wall clock never goes out of style: https://amzn.to/33k87OE I've never lost my love of floating shelves because you can dress them up with your favorite decor items and when spaced out well, it never has an overwhelming look. #affiliate link

Just moved in to our rental, they won't let us paint so looking for help with window treatments and pictures. The room open to the dining/sitting room

You don't need to fill each nook and cranny. Negative space theory says the eyes need to rest sometimes. For the large wall, consider a local artist or art gallery. Also, some colleges have their own art galleries. If you don't get anywhere there, I've gotten some beautiful prints from art.com. They have a wide selection. I just buy the print from art.com and then I get it framed locally with plexiglass (safer and lighter than glass, and thus easier to hang). You don't have to be in a hurry to fill the walls. Wall decor is highly personal and each person has their own taste. Wall decor can even be a conversation starter! Besides framed prints, you can also get 3-D pieces such as an iron piece, tapestry, or a plate grouping.

never a fan of "niched wall"! but since you mentioned this home was custom, which means he designed this sort of walls. so it is what it is. my best advice is forget about generic acrylic "art" pieces you showed here, try sculpture - one piece that fits with the entire niche. if he can have a custom home built, he can find a local artist and commission one that fits. if it is not possible, try groups of paintings and/or photographs (aka collage) that are 1) interesting 2) original 3) and PLEASE NOT the same size and the same theme - they only make the all set up look cheap and contrite. another idea - try floating shelf. this website has many great examples of display using floating shelves. (i am attaching a pic of my own living room i just did. it is not a good one. but i used floating shelf idea for the wall on the left side.) last but not the least, paint the back of niches a color ... your friend likes grey. try a medium grey.