I am not a scientist or doctor, only a member. So I can only speculate and make suggestions.
Even though you would like to take more, perhaps genetically you simply do not need more then the 6000-7000iu.
Perhaps you really need to square away the magnesium deficiency before even trying higher amounts. Sometimes I have recommended taking magnesium for a week or two before even trying the Vitamin D. You can try taking even more magnesium if you want to. Worse case scenario is that you will get “loose bowels” and then you have to cut back on the amount you are taking.