How about a strip of stainless steel? I would make it tall or a piece of pretty glass in a color you love. You can buy glass at Hobby Lobby fairly inexpensively and cut a tall strip. You could use a few different colors and foil and solder them together. If you use glass, use white thinset to adhere it to the wall and don't use a notched trowel or you will see the lines through the glass.

What about pvc beadboard wainscotting? Since we can't see it, I don't
know if it would be too difficult to fit around the pipes and behind the
toilet.

This is just an example and it seems easy to use, and there are others with great reviews. Vintage peel and stick backsplash

Peel and stick wallpaper Wallpaper handles water from sinks, we have some in the guest bath.

Check out what I did in our bathroom. Some mosaics with a trim. You could get fancy with colored tiles.

This is adorable but maybe too much for your situation. I wonder if you could use a piece of marble threshold as a backsplash! Or would it be too deep behind the faucet??

Or a strip of tile across the whole sink wall, not just over the sink?

Thanks all! These are good ideas and will give me some direction here. I do worry that marble will be too fancy, but I do have a slab left over from a few remodels ago just sitting in my back yard... :)

Would it be tacky (tackier) to use a chunk of flooring there? The floor is 8x16 gray slate. I have a huge amount left over due to a GC who didn't math well, some in much larger sizes that we used in a larger room. I could cut one down and stick it up. I even have a tile saw! (I'm planning to DIY whatever I do. Our town is still pretty covidy.)

Even if it weren't tacky, would a rectangle of gray in a bland whitish room stand out too much? Guess I can chop and prop one up and see. (We have so much extra we considered using it for a patio and pool deck. Too hot!)

I will have to get some better photos.

I think I'd paint it with high gloss paint in the same color as the rest of the room. I have had high gloss paint behind my stove for years and it wipes off easily, no signs of wear, and I cook.a lot!

You're probably right Sheesh. I should just do the whole room, really. I didn't have issues before the new paint job. And the new paint job isn't great anyway. But it was a pain to paint, even though it's one of those rooms that doesn't require a ladder because you can reach everything while standing on the toilet. :)

Aha! I get trying to avoid a difficult pain in the neck job with a simpler solution. Well, if you find that solution, do it!

Or you could just repaint the sink wall in the high gloss...

I have used BM pearl finish in my bathrooms, and my painter says the Aura BM paint is the best for bathrooms. So if you do repaint and you don’t want high gloss, maybe try the Aura in pearl finish.

When I did my bathroom, I had painted white beadboard installed. Due to my concern over the paint needing extra care behind the sink, I placed (leaned) a very large antique white platter behind the faucets which covers the entire area. The platter has a simple edging design in my greenish-turquoise accent color, looks great and is easy to clean. It echoes the shape of my sink, a console sink on legs. I always intended to swap out holiday themed platters, and I did initially, but haven't in a few years as I guess I like this one so much.

I like the idea of a rustproof metal backsplash. I have a couple of vintage hotpads that are made from quilted metal and want to mount them on the wall to use as magnet boards.