You will have to support that larger overhang and to be honest 12” is really all you need. I find it funny that someone short would design a 60” wide island , I am tall and would not go wider than the 54” I have so that when I clean it I don’t have to walk all the way around. As for making the counter lower than average your resale value will be affected for sure.I think you will maybe have to look at quartz jumbo slabs , They are limited in which ones come jumbo and sometimes the actual logistics of getting them into the space is a problem. All of this should have been addressed by your designer before construction began. There are steel supports for the overhang so you can avoid the corbels that are IMO ugly and take up seating room.

I would completely detach a separate chop/ prep spot if this is what you want, or add a pullout toe kick booster and give yourself a lift when prepping. An island over ten feet is a nightmare for MEGA slab selection. Post your layout. It's too darn big. And nobody needs an 18 inch overhang, either.

The island has been built. I made it that large so that I could best use the space in the kitchen, which has an L-shaped counter on two sides; one of the other sides is the dining room (open layout, timber frame), and the other is the hallway from the garage. The long counter has the sink, which is centered in a 7' bay window (views west over the Hudson valley to the Catskills), two small upper cabinets on either side of sink (glass fronts, lit), and a wall unit containing steam oven and wall oven. The short side of the room has a window with a pass-through to the screen porch; that counter has freezer drawers as well as regular drawers (all the lower cabinets, except the sink, are drawers). Next to it is a 36" wide all-refrigerator unit. The walkways are 48" wide. If I had made the island less wide, there would have been an awkwardly-wide hallway. This way, I get drawers on both sides, and storage, including a crystal cabinet on the far side of the island. The cabinets are made from cherry and spalted maple cut from our land.

I chose to make the island 34" tall because I have chronic tendonitis in both elbows, so it is easier for me to chop, roll out dough, knead, and stir at more than a 90 degree angle. There is plenty of room at the side counter for helpers to chop, if they want a higher counter; in addition, a thick chopping block would bring the level up. This is our retirement home, and we expect to live here for twenty years or so, at the end of which time it will need to be redone anyway, I reckon.

So I will reduce the size of the overhang to 12 or 15 inches, which means that the soapstone I looked at will work (biggest slab is 62x124), assuming the fabricator can incorporate a steel support. I talked to the foreman today, and he thinks putting a layer of wood under the soapstone, to bring it to the 34" I originally planned, will work.