This is another topic really but I will provide a rough answer.
I don't think there were any wide genetic differences between the early Kermans, C-Group Nubians, and the later Pan Grave - these people descended from the same Nubians -- all Cushitic speakers, and I would even say they were Eastern Cushitic speakers (later Kermans language-shifted with probably genetic influence from Saharans). Their pottery was a variation of each other. Their burials also showed various affinities (they had different trajectories that influenced each other several times over). With regards to subsistence, they were agro-pastoralists who originated and orientated with a heavy travel mix, never did they fixate completely (they did retain pastoral ideology even when farming was emphasized) other than to adapt and mix economies according to their needs. One cannot interpret this as an ethnic difference. Kermans lived in a region that was greener so they did farming more while pan Grave and C-Group stayed in more arid areas to the north and northeast. However, as the Kadruka cemetery showed the pre-Kerma, the groups there were mixed economy as things generally went:
"The picture is still incomplete, and relies on a limited number of discoveries, but points to the occupation of the land by an agro-pastoralist population that fills a local void preceding the Kerma civilization (Fig. 8.1)." Emberlling & Williams, The Oxford Handbook of Ancient Nubia, (2020), p. 145
That quote refers to a cemetery of what led to the Kerma, so pre-Kerma.
Here is another saying that the material burial culture had a common origin reflecting what later became differentiated broad peoplehood reflected in the funerary ideology:
"Moreover, cattle were sacrificed in great numbers during the funerals, from several dozen to even several thousands. Such a change did not occur suddenly, as can be observed in the cemeteries of the Pan-Grave and C-Group cultures of Lower Nubia and the Kerma culture of Upper Nubia (Chaix 2000; Hafsaas 2006; Bangsgaard 2010, 2013, 2014a; see also chapters by Liszka and de Souza and by Hafsaas, this volume). The common practices and similarities in material culture of these groups belonging to the Middle Nubian Horizon are generally explained by a shared cultural foundation and origin (Gratien 1986; Bangsgaard 2014b). Undoubtedly, these partially contemporary cultures fall within a deeply rooted tradition of animal deposits in Nubia, as they pursued the practice of animal inhumation in pits including sheep and cut body-parts of the animal inside the grave shaft. However, they moved the ritual use of animals to a new dimension with ostentatious deposits of cattle skulls in the open air."
It is very complicated to be honest, because the concept of ethnic group is not universal:
"It seems relevant to analyze animal remains discovered in archaeology in this way, while keeping in mind that ethnology always prompts to interpretative caution, especially when it reveals the complexity of funerary rituals, the multiplicity of practices and their motivations, and the diversity of representations of the hereafter."
Those people might have seen each other as the same people but had distinctive practices and ideologies very different from how we view populations today. Similar to how pre-historic Arabians had a broad conception of the Arabian population. So clans/tribes are not akin to ethnic groups but neither was the broad Arab conception either. More like race (although that is really a term defined very differently here than how people usually think of it). That is just an example, since those people likely were cognizant of who they were and how they related but had unique ideas of the broader.
There are also different layers to this. Both Kerma and C-Group might be Kushite; Pan-Grave arrived later and is probably a variation of those, like an offshoot.
I generally have this view as posted in the previous drop:
"With the term Pre-Kerma is indicated a third culture, originally identified in the Kerma region and dated from the end of the 4th millennium to the first half of the 3rd (Honegger, this volume). As such, there seems to have been more than one culture contemporaneously present in the region, when in fact that is not the case. The discrepancy encountered in the archaeological record should be read as an expression of an intracultural variability that, as previously stated, needs to be addressed on a supra-regional level, taking also into consideration the alternative type of socio-economic structuring and power organization that characterized Nubia. In this paper, I retain the term “A-Group” only to define the political entity that arose at the end of the 4th millennium bce." - Emberlling & Williams, The Oxford Handbook of Ancient Nubia, (2020), p. 127
In the previous stages, pre-Kerma and A-Group had similar pottery. We're not talking about different people but the same people expressing variations:
"As regards the typology of the pottery, the differences between the Pre-Kerma and A-Group are rather subtle and it is difficult to interpret their significance.
Overall, they represent a single cultural horizon, but it could be that the differences observed reflect the existence of several (tribal) groups, similar to those described at a later date during the expeditions of Harkhuf (about 2287–2270 bce; Török 2009:69–70)." - Emberlling & Williams, The Oxford Handbook of Ancient Nubia, (2020), p.147
"It is possible that the few sites in Lower Nubia represent persistence of the A-Group in the region, and that the two entities, A-Group and Pre-Kerma, represent at that point in time a single cultural entity (Gatto 2011a). We can conceive that the commercial routes, as well as the population movements from the south towards the north, in the wake of the growing influence of Egypt starting in 3000 bce, led to a level of cultural harmonization between Upper and Lower Nubia. However, we need a greater volume of data for this period to be able to be more precise about the situation." Emberlling & Williams, The Oxford Handbook of Ancient Nubia, (2020), p. 148
Well, it is not just homogenizing as if there was cultural diffusion, which it was. The point was that these people descended from the same late Neolithic Cushitic pastoralists people in the first place.
One cannot rely on Egyptians to be precise. But one thing is important to note, Egypt emphasized that Kush had allied with these other groups to attack them, likely reflecting a supra-regional conception of affinity and some of those existed as a broad economic and cultural variational cluster instead of hard ethnic lines of today people:
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This was from an inscription in Elkab during the 17th dynasty by the Sobeknakht governor:
"Kush came ... he had stirred up the tribes of Wawat, the [islands ?] of Khenthennefer, the land of Punt and the Medjaw"
From this, we even have evidence that they would even band together on military strikes, essentially superimposing on the related affinities they had on a political and military projection.
You had C-Group Nubians buried in Kerman cultural contexts, according to archeology:
"The origins of this group, which occupied Lower Nubia from 2500 bce, continues to be unknown, and its presence in the Kerma cultural context could be indicative of contacts and a certain freedom of circulation between the two groups." - Emberlling & Williams, The Oxford Handbook of Ancient Nubia, (2020), p. 149
Safe to say, these hard lines of ethnology are not warranted.
By the way, it is not totally unknown as the source claims. C-Group Nubians, though a later re-introduction to Lower Nubia, was probably A-Group moving south and mixing with Kermans, then moving toward Lower Nubia again. The affinity they had with the A-Group was evident, though they had distinct changes
