The toilet will not have high humidity, as in bathrooms, so I would simply use decorative plaster on the walls so as not to add another type of tile or not to use the existing one. To decorate the walls, I would make joints between different segments of the wall (black lines in the pics). Also, I would use the same wood on the column as for the hallway and living room; this will decorate the space, but laconic looking.
Also, I would add ceiling lighting. In the toilet the ceiling is higher than in other rooms, this will give it an even more flying view.
The second option is grey. Here I used chipboard for the walls.
In general, you can ask furniture companies what the price of such treatment for the whole room. Maybe they also have a white material with a beautiful texture like concrete. You will be able to compare prices. In theory, this should be cheaper than making the whole toilet in tiles, but more expensive than decorative plaster on walls, but I can’t say for sure.
I tried to leave the existing toilet and sink, and also changed the sink to vanity to show how the appearance of the room changes and how the vanity affects the design of the wall.