What causes some fish to get texture like mashed potato suddenly?
You have already diagnosed both your problem and the solution. Your intended cooking method is to poach the fish in ghee, you are not poaching it (for poaching, you will have to immerse it in a generous amount of liquid, in this case ghee) and thus it overheats.
You should either accept that this dish takes several kilograms of butter, or choose a different preparation method for large fish. What you cannot do is take away what makes the method work (the ghee) and then still expect it to work.