Try canned pumpkin pie filling that has nothing added to it. Some of them have sugar & stuff added but some cans of it are pumpkin only.
It's very high in fiber, it's not harmful to a sulcata & it can help make that rock pass on through in a few days.
If it doesn't work ask a vet about lactulose. My vet has been prescribing it to several people for impaction. You need a vet to prescribe this & give you the correct dose for it.
I have a box turtle who swallowed small pebbles in the outdoor pen this summer & she did get impacted. The vet prescribed this stuff & it worked.
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PHRatz