We can't afford to do that quartz as a full backsplash on both walls.

From what I can see the options are either doing a full backsplash under oven, then drop down to 6" on both walls.

Or do full backsplash on oven wall, then drop down to either 18" or 6".

I've seen some full marble-lookalike backsplashes that are really great, but I don't know if it's going to have the same effect here - or how to reconcile the transition to sink wall.

Beautiful material! Sounds like you are planning a horizontal line across the refrig/range wall at 18" backsplash height, correct? If, in fact, your range has a tall back control panel, I would suggest taking the backsplash up to the height of the hood. Will be prettier and more dramatic. Am unclear as to what the items are on the sink wall - ? A sliding window at the sink and 2 other fixed windows? You could use the same material or make an accent here of small mosaic tile that matches the coloring of your countertop/backsplash material. Something similar to the "bands" of tile in this bathroom. Could be a lovely variation on texture.

Sonoma Tilemakers has some good variations that might work with your countertop material.

https://sonomatilemakers.com/product/vihara-12x1-14/?attribute_pa_vh-brick-color-palette=puka-sk-brick

@Deb - Thanks! We were planning on taking it all the way up to the range hood. I really like the idea of using a white mosaic tile on the sink wall! The tiles you linked to are stunning. The sink wall has one window above sink. The two rectangles were hypothetical artwork. :-)

After thinking about it some more, we've decided against the tile so we still have to figure out how to wrap around the backsplash.

Our key questions now are:

1 - Would the 18" backsplash on sink wall be overwhelming?

2 - Would a full backsplash turning into a 6" backsplash look strange?

As an alternative or accent border, you might want to
consider an easy-to-install adhesive approach for wall tiles that’s
less costly and time consuming, yet stylish and enhanced with 3D subtlety.
Sometimes the small details can really make a design (and it doesn't look cheap, it's very nice). Check out a few ideas
here https://www.houzz.com/projects/1262017/kitchen-backsplash

Thanks! I think I'm going to wait until the cabinets are installed to get a feel for full vs partial backsplash on that oven wall. I suspect my decision will be clearer once I can see it in real life.