Indirect lighting can be placed well behind almost any surface that allows light to fall off at the edges or on the surface. That means it doesn’t have to be mounted flush. Maybe you’d like to imagine a piece of frosted glass with mirror holders. I once saw something like that in onyx with a gold frame at the home of an enthusiast of opulent design, yeah.
If it’s an LED strip, you might want to think about heat dissipation and possibly mount the light strip on something that can conduct heat away, especially if you enclose it.
Glare-free is important; two heights should be considered because it will be viewed while sitting in the bathtub.
I wouldn’t add a decorative element because otherwise the wall would look unproportional.
I might also copy a nice downlight into the photo. Under a downlight, I might place a beautifully proportioned display piece, a sculpture, a plant, something you have a connection to.
Have fun designing!
Gabriele