I think that what you meant to say is how the Egyptian nomarchs built elaborate tombs (though not pyramids, of course) “until” the “gaining” of utter authority by the central government in Egypt. This all took place during the reign of Sesostris III, in the 12th Dynasty. The new trend in Egyptology is to suggest that this curbing of the power of the nomarchs took place slowly and gradually.
However readily the “slowly and gradually” theory is applied to almost anything in ANE studies nowadays, it absolutely does not apply to the curbing of the power of the nomarchs. The only thing that may have happened slowly and gradually is the final elaborate burials of the ones who lived the longest. Either 1 or 2 of them are attested as having been buried during the reign of Amenemhat III, but not later.
Back to Nubia, I studied Nubian archaeology under one of the curators at the ROM, who has been digging at Meroe for many years. Although he could answer your question much better than I, as most of my study of Nubian archaeology focused on the MK (Egyptian) forts in Nubia, I would doubt that the proliferation of pyramids has anything to do with the weakening of the central government’s authority.
In fact, the pyramids at Sedeinga mostly were built during the 1st century BC, and during the time of the kingdom of Kush/Cush. This was not a time of governmental weakness, but of strength. In my mind, the pyramids—if anything—reflect this time of strength. Again, this is not my era of specialization, but I would be greatly surprised if the truth is anything different than what I am suggesting to you.
Hoping this helps,
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