If you are doing a quartz backsplash and all of that existing stone will be demo'd out anyways, one solution that we love the look of is removing your current window sill / bottom casing, and replacing the bottom and sill ledge with the quartz. That way your window looks a little more intentional and less like you were trying to work around it. Here's an example:

Agree with LS - we did similiar approach with our slab BS to keep it simple

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