Do you like patterned tiles at all? Doesn’t have to be garish and you can keep the pallet simple. I’m thinking tile the whole wall, the same width as the vanity unit, up to the ceiling, similar to first picture. Or if that’s too much, then just tile the space from the sink to the mirror. I would raise the mirror a little so that it doesn’t get too splashed.

Are you saying the glass splashback is the wrong size because it doesn’t go all the way up to the extractor fan? If so I’d guess that they’ve installed a standard size as opposed to a purpose made size. If so l would just take that one down and get a taller one made. You could always have the splashback all the way across the wall. l don’t think it needs tiles too. You could add a tint of colour with the emulsion,

I have tiles on one wall by the sink, and mirror in a darker area. I also have wallpaper over the other (non-cooking) side of the kitchen. It's great!   Do what works for you.

Just something to consider. This is only a digital render but try to consider the colour and warmth or coolness of the upper cupboards against the colour and warmth or coolness of the worktop. If one reads a warmer than the other and they are equal in terms of lightness then they might clash. Cool worktops and off white warm tiles or cupboards are tricky. Additionally, any under counter lighting - get the best you can, with the highest CRI rating. LEDs vary greatly in light quality and temperature.