Ok, I had to look up what Japandi is and may not totally have the aesthetic down. But here are a few items that might combine to be what you are looking for. It's fun learning about a new style and I can see the attraction to Japandi.

Floating sink in wood could bring in the Scandi look. You'd have to see what wood looks best with your flooring. Use black faucet and black for other fixtures in the room including lighting.

Ambra 24" Wall-Mounted Single Bathroom Vanity in Nordic Oak with White Ceramic Top · More Info

VIGO Noma 8" H Single Handle Single Hole Bathroom Faucet, Matte Black · More Info

Square Matte Black Toilet Roll Holder · More Info

Hutton Wood Framed Capsule Mirror, Natural 24x36 · More Info

On the walls I like the idea of grasscloth to bring in a natural element. I'd prefer a neutral, natural color so the black fixtures can pop off it. In this case I'd use a black mirror too. If you use a greyish or blueish grasscloth you could hang a natural wood colored mirror like above to contrast the walls.

Imitation Grasscloth Wallpaper, Ochre, 1-5 Bolts · More Info

Imitation Grasscloth Wallpaper, Gray, 1-5 Bolts · More Info

Put a simple piece of artwork above the toilet that has a lot of negative space and brings in some of the colors that you like.

Scandinavian Clarity 2 Abstract Ink Print Wall Dcor - Multicolor · More Info

I'm not sure what your lighting situation is but I would go minimal and contemporary.

A couple of points I never like a hand towel in a PR I do not want to dry my hands on a towel everyone uses so think of disposible towels for sure . If Japandi is your style then IMO go very minimal and certainly not that sink I love wallpaper in PRs but can you post a pic of the flooring since that will have an effect on what you do .The mirror you have is too big for a pedestal sink and needs to be nicer . I do not think you need to have the toilet brush in the space . The lighting needs to be good in bathoroom so can you post the fixture you have? I like LEDs 4000K in both kitchens and bathrooms.

I like the idea of grass cloth.
I especially like it with green tile. Maybe you can incorporate somewhere. (See photos below)

New mirror

New vanity

New flooring - can you show what you picked out?

Wall treatment.

This is a digital rendering of what the floor will look like. Trim will be white (I am painting it while floor is being installed). Also this is my current lighting situation 🤢

I’ve toyed with the idea of getting rid of the towel ring and just using paper towels, but I don’t have a place to put a nice tray to hold them.

Here are some options for the wall:

ESSENTIALS 12x3.5 - Pure Silk

PATINA 12x3.5 - Champagne

NEW WAVE - Sutra Black

GLASS ARTIFACT - Stratos

CRESCENT INTERLOCK

Spindrift Glass - Stratos

ESSENTIALS 2x1 - Oceania

Below are some of our powder room designs we have completed for clients. Perhaps looking at them will give you some design ideas on what you might like to do in your own powder room.

Golden Renovation: Powder Room · More Info

Highlands Townhome Renovation and Decor · More Info

Observatory Park Home Renovation & Decor · More Info

Evergreen Design and Decor · More Info

Cherry Hills Residence: Powder Room · More Info