Behind the sink and under the windows I’d use a white Corian solid surface. For the range wall and the coffee (?) nook wall I’d do a tile in a matching white with a pretty shape

hard to tell exact colors from the pics but seems like you’ll want to unify the cabs and counters while keeping the floors in mind - and not too busy bc the counter has some movement, I think it could be tough.

If it were me, I might skip a backsplash except for a stainless panel behind the range and perhaps just a strip of low key glass behind the sink to protect the wall, or counter material under the windows along that wall would work too (I’d prefer that if my fabricator hadn’t finished the job yet)

I'd bring home some simple subway style gray tiles.....skip the whites.....dark hardware/ dark grout lines in floor/ some dark in the counter.....heres an example but I'd prefer a matching grout to the gray tile of choice....

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Keep it simple. Let the other design elements sing! Go with the average 4" type filling the gap under the window with your counter top. Your counter is rather busy, but since there is very little actually 'back' in your kitchen I think you could use the counter top throughout. The SS behind the stove would work. but adding a new element seems like over kill. There is no need for added material in the eating/buffet area. That won't be a water or prep area. Paint the same as the wall.

Based on your photos, you have three competing undertones - the warm cream cabinets, the grey, white cool counters and the peachy floors.
I would use a subtle mixed finish subway tile that ties the three undertones together. Definitely bring samples home and view them in your space,

I think a simple corian 4" backspalsh and the stainless behind the range and be done the cabinet counter combo is not ideal so IMO do not add another material and color to compete and IMO all stainless hardware would aslo be a good idea.

Would suggest that you replace the flooring.

Dee and Norwood are in line with my thinking. You have a clash of floor and counters. Best to post pictures of tiles in your space. This will be tough if the floors are staying.

Black subway tile might be an option, but the floors definitely clash.

So what were you going for because you have warm white cabinets that don't play well with the new countertops. The countertops don't work with the flooring either.

I wouldn't add another finish into this. Use the countertops as your backsplash and at least have two items that work together.

For color you might consider trying to match the background of your stone. You could choose instead to match the beige of your cabinets, but in this case you’ve already created a contrasting worktop. You’ll get better visual flow sticking with your stone colors.

I with those who have mentioned gray.. I would do a very very light gray. The white of the counter fights the white of the cabinets. Plus you have the floors.

I do hope you will consider redoing the floors as it appears to be a separate room?