The slab should stay quiet
Japandi doors are flush or nearly flush. A big raised panel, a row of colonial lites, or a glossy paint fights the room. Light oak, ash, and warm white are the usual finishes. Black works when it is matte and used on one or two openings, not on every door in a small plan. Grain, if you show it, should be straight and calm. Busy cathedral grain reads as rustic, which is a different style.
Glass, when you use it, should be a single field of frosted or reeded glass rather than a grid of small panes. The grid is a French-door idea. Japandi would rather have one quiet sheet of light.
Hide the hardware, or make one line of it
Concealed hinges and a small black or satin lever keep a swing door in the wall. Frameless jambs go further and remove the casing. That is the Japanese side of the mix: the opening is a gap, not a frame. If you want the door to slide, a pocket door is closer to this look than a barn door. A barn track is a deliberate black line. Use it only when that line is the one decoration in the room.
Do not add a second metal color. One finish for hinges, pulls, and the faucet you can see from the same spot. Mixed brass and chrome is the fastest way to break the calm.
Repeat it through the house
Japandi fails when the bedroom is oak, the bath is gloss black, and the closet is a white six-panel. Pick the wood or the white, pick the one dark accent, and order those doors together so the finish is from the same run. Heights should match. An 80-inch door next to a 96-inch door in an open plan looks like two houses.
Casing, if you keep it, should be flat and the same color as the wall or a step warmer. Thick colonial casing around a flush slab undoes the door.