There are tile stickers you could try. I wouldn’t think they are very durable to be honest. You could probably tile over the splashback with tiles to lighten the effect?

I personally love tiles. I think they add interest. However I had to google Wow Fez tiles as I had no idea what they were! So they are Moroccan hand made style tiles and you are wanting plain white ones? Is that right? Are they like the ones in my second pic? If so, I think they are beautiful. I think metro/subway tiles are a bit boring unless they are an interesting colour. I would tile behind the cooker and up to the window sills. However some people have just an upstand in the same material as the worktop (picture below) and use a washable paint on the rest of the wall. It boils down to personal taste.

Yes, me! I have a bronzed mirror throughout the whole length of the kitchen run. I love it, don't regret it. See here: https://valleyhouseinteriors.com/2021/07/30/new-open-plan-kitchen-living/

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,