Kitchen backsplash, which material?

We were thinking of a self-adhesive film with a design

On Q2 plaster? I would do that only in a rental apartment with turnover. It doesn’t stick well, you can see the uneven substrate through it, and the material wears off at the edges. Better to just paint and seal.

Granite on the wall, identical to the countertop. Height up to the wall cabinets/[Abzugshaube].

We do not want granite on the wall because it would otherwise become too dark and expensive.

We just want to have a colorful motif to bring some colors into the kitchen.

I only have experience with "eternally sticky" in the kitchen with the Pril flower. Adhesive foil on the wall works if the wall is well prepared for it; the foil specialists can say more about that. We covered a door at the company. It sticks great, but working with the surface was very challenging for beginners. If it's foil anyway, I'd have it done professionally.

Acrylic would be another option that might be a bit easier to handle. If necessary, you can also drill additional holes yourself.

Slides come in different thicknesses and qualities that seem to be quite decent, but then again cost almost as much as glass.

I also prefer glass and recently installed a motif glass splashback for us. However, we decided not to cover the entire back wall, but only a "small part" in the wet area. Along the entire length, there is a baseboard as a "wipe strip" with a height of 8 cm.
There are hundreds of different motifs in various sizes for relatively little money. I didn’t want to set mine on silicone dots (which I had before) and now simply placed it "on top" of the baseboard and "attached" it behind the wall cabinet at the top. A silicone joint at the lower edge will hopefully be completely sufficient.
There are also ones with eyelets for hanging...

After a while, the splashback can be easily removed at any time and, for example, replaced with a different motif.
For this reason, I personally also recommend not covering the entire wall, but only placing "spots" and keeping it as flexible/exchangeable as possible.