Is it recommended to have a bathroom and toilet with few wall tiles?

Take at least the weakest possible grout-tile contrast, otherwise you will ruin the friendship.

I let my wife lead on this. She wants tiles as large as possible, with almost no grout (i.e. rectified) and preferably in the same color tone as the tile. (That won’t be so easy, because the grout changes, or darkens, over time)

The larger (and especially square!), the more dominant every interference of the grout rhythms becomes, which cannot be aligned.

She even wants to lay the tiles in a "running bond" pattern to make it look more harmonious. Straight lines. And also the same tiles (in the same format) for the walls. (where the same tile comes in the formats 30x60). For the floor, there are 80x80 or 90x90.

It sounds cliché... But I would talk to the tiler, they usually have seen/tilled hundreds of bathrooms and usually give good advice.

It’s not that cliché. Actually the better way is for wife and tiler to talk to each other. Then it depends who is more dominant in the conversation :-)

If it doesn't necessarily have to be tiles, there are other options. We don't have any tiles in the house at all.

For example, the guest bathroom that is not yet finished.

We don't have any tiles in the house at all. As an example, the guest bathroom that is not finished yet.

Not only the bathroom, the post itself is apparently not finished either: you didn't describe exactly what can be seen there instead of the non-tiles. Is that latex paint or some other garage floor coating?

Hatta right :) It is a coating in concrete look. I am not sure if I am allowed to mention names here. The substrate is prepared like for tiles, then applied twice, sanded and sealed.

She wants tiles as big as possible

Large tiles make the room look smaller if the room is of normal size. Only in spacious rooms does the size of the tiles give a "large" impression. The reason it sometimes looks awkward in rooms of normal size is that only one and a half or two tiles fit side by side. Our brain immediately recognizes this for what it is, and it doesn't look like "a lot" or "big." I have yet to see a nicely tiled bathroom floor with such large tiles.

She wants tiles as large as possible, with almost no grout lines

I’m putting a big question mark behind whether fear of grout is a good design guide.