Real Granite or Real Marble next to quartz will not look good as real stone next to fake always makes the fake look, well, fake. Tile is going to be in and choose the tile AFTER cabinets, tops, floor, etc. are all finished.

If you want the slab backsplash, then your counters need to change if there's not enough to do what you envision. There is no cost saving if you need to get another slab material for the backsplash. Post photos of your space and the selections, why are tiles not a possibility?

Perhaps the company that manufactures your countertop knows of another of their quartz which is the same background color, and you can do a very simple backsplash in that background color.

IMO go get real stone and enough to do what you want or get the fakstuff form another supplier that has enough BTW can't IMO put plastic behind a range anyway.