You need functional alterations for a better design before you delve too deeply in materials choices. That’s too long and spread out. It’s costing you money and function both. That waterfall is $$$ dysfunction, and the island doesn’t work without a prep sink. Hire a Kitchen Designer.

Or, add a prep sink under the window between the fridge and range.

In addition to the comments above, the aisle/space around the island looks EXTREMELY tight from the photos. If you post a measured floorplan you'll get much more specific advice.

The kitchen as pictured does not suit a waterfall island. Waterfall islands are also difficult and expensive to get done right. The veins have to flow exactly on all sides, which means extra slabs, the sides have to be mitered perfectly, and where the floor and Quartz meet has to be done correctly. Don't forget the electrical requirements for an island, which are made a bit more difficult with the waterfall sides. This is all for a look that does not appear to go in the kitchen you have pictured. I'd spend that money instead on better appliances, or any number of improvements and enhancements a kitchen can have.

Edited to add: A backsplash that you asked about should be chosen absolutely last, after everything else is installed. Everything looks different once installed, and if you choose a backsplash too early, you will make a mistake.

I think you have some great aesthetic choices in the works. But pause those for a bit to get the kitchen design that is worthy of all of the great new stuff! Kitchens that look great, but are difficult or unpleasant to use, don’t overcome that deficit. A beauty queen still needs to be able to answer the essay question adequately! That’s what lasts!