I forgot to mention that I cannot paint the room

A freestanding indoor plant, or a great faux one, would bring a new element in under the towel rail. The bright green of the soap bottle actually caught my eye - using lime green in towels could be interesting if you wanted some colour. Alternatively staying with neutrals or naturals ( thinking the pattern colour of the shower curtain or eucalyptus) would work.

There’s a site called Polished Jar that has very nice soap, shampoo etc dispensers - they are a nice finishing touch - I could see a large framed B&W photo on the back wall of the toilet room

FWIW I would probably pull in hits of black to amp up the drama a bit

Have a hay day in home goods or the like☺️💖😅

Add an art piece over the toilet and then once you had hand towels, and bath rugs and possibly a basket with toiletries and the space will feel very welcoming. Beautiful so far….

I love original art and shopping for it but I saw this at a Lazy Boy store and it was perfect for our guest bath so bought it and still love it….

Your room is so neutral, you could add any color you love, in art, towels, rugs. Or if you like the monochromatic look you have, then make similar choices. I'd want a rug in front of the sinks; I like plush, washable. Art in the toilet room, either above the toilet or on the large wall (which I'd prefer so I could enjoy it while using the loo, but I guess men also can enjoy what's over the toilet). And even though mirrors would be an odd choice for the toilet room, I'd be tempted to try some (maybe a collection of antique mirrors) on the large wall across from the window. I'd love to have the view out the window reflected so you could see it from the toilet. (or maybe you can already see out the window when seated)? I like the contrast of the vintage ornate mirrors with your minimalist style, but that might not appeal to you.

or one that's more like a piece of art:

or one big statement piece (not this color)

Art.com has great choices of artwork that is reasonable. Changing out that mirror would help as well as some colorful towels/handtowels to pick on the artwork you choose.

@DaneAustin Interior Desgin where can I buy the rug in the top image that you posted