(edited for clarity, removed addendum & comments - see final comment by Mark C. Wallace)
I'm wondering if a more comprehensive picture of the extent of Kushite horse breeding and trade has emerged as part of this "ascendant Kushite Dynasty" and has been summarized in recent scholarship. ... Since evidence has come out rather piece-meal over time I'm hoping to get a bigger picture of the Kushite horse "industry" (if it may be called that).
I think the idea of horse industry - as OP indicated - may be overstating it.
In fact, I'd go further and ask: was there significant trade of horses during Kushite period (25th Dynasty), as opposed to gift-giving (prestige goods)?
1. Horse Trading vs Prestige Goods (Gifts)
After Megiddo (mid-15th century BCE), Thutmose III seized 2,000 mares and 6 stallions as war booty. This was the start of the proto-Arabian horse in Egypt. Prior to this period, there was no significant iconography of horses or related activity. Even during this period, Egypt struggled with introducing horses into their world and therefore, I believe the stock-breeding was to maintain/grow their horses for warfare. Trade/export would diminish this important livestock.
From 13th to 10th century BCE, the importance of horse-training grounds of Piramesses was its strategic military staging post (vs the Hittites & Shansu). In other words, breeding and training horses for warfare.
From 8th century BCE, given the unique horse burials at el Kurru, especially the horse accoutrements buried together, it is clear that horses were were still very highly valued and clearly the King's prerogative. To date, there has been no discovery (however, see quote above from Heidorn) of private horse-breeding during this period. In other words, horses were high-value, status-enhancing, livestock only fit for royalty (i.e. prestige goods). Hence, it would be surprising to find Nubian horse trading activity.
The recent discovery of the "Tombos horse" (OP's picture) further confirms the high-value of the horse:
A tomb burial for the horse suggests that the animal probably played a significant role in its owner's household, and was more than a mere beast of burden, while the iron bridle piece found in the tomb — an expensive and rare item that would have been made specifically for the horse — further helps to establish its elevated status, according to the study.
Source: This 3,000-Year-Old Horse Got a Human-Style Burial, Live Science, April 26, 2018 (emphasis mine)
2. Large Horses
There is another matter of what horses were being bred (i.e. the breed)? Why was it so attractive to the Neo-Assyrians? In some sources, such as Heidorn's 1997 article, the argument was that these were large horses.
From my comments (which is removed): On the "large stature" of the horse, is there any discussion on the 'breed' itself because, to my understanding, it should be smaller/lighter type, i.e. proto-Arabian. Especially if there were seized from nomads of Near East. Horses buried in el Kurru were facing NE, most likely Bactria/Sogdiana or Black Sea? Weren't the imports from that direction, i.e. transiting thru the Near East? (Excavations from horse burials shows horses of steppe nomads & early Chinese facing east generally -- NE,SE,E, direction of rising sun). All questions that cause me to think it was more than the horse (breed) itself, or at very least, perhaps the horse equipment was also highly valued.
I still have no idea why "large horses" and do not have breed info on Kusaean horse (from Nubia) - see below (Postgate's quote, point 4.2) on the naming of this breed.
3. Trading vs Breeding
After discussion with OP, and subsequent edit of question, I believe there is a distinction between a trader/trading (export industry for horses) and breeder (location best-suited for breeding). In other words, Nubia (Kush) was probably a good place for breeding horses. However, that does not mean there was horse trading involved.
OP makes a point that it was highly-prized (final para, from Heidorn's 1997 article) as chariot horses. I see that as status symbols and, in my mind, the Kushite horses were proto-Arabian, so I don't understand why "large horses" would be prized (stamina goes down in the heat) unless they were status symbols (hence my "prestige goods" point). Of course, OP could be correct and I'm wrong.
In any case, here's two paragraphs from Sandra Olsen, "A Gift from the Desert: The Art, History and Culture of the Arabian Horse" (International Museum of the Horse, 2010), pp.52-3 (emphasis mine). It establishes Nubia as a good location for breeding horses:
It will be recalled from discussion in the previous chapters that the horse was domesticated in the Eurasian steppe, definitively before 3500 BCE, and to have made its way to Syria and Mesopotamia by around 2500 BCE. Although domesticated horses would have been able to enter the Arabian Peninsula at this time, the climate was probably already too severe in much of the region to easily sustain this northern species. It would take another 1,000 years and the development of the proto-Arabian breed before there were horses adapted to such extreme heat.
Domesticated horses arrived in Egypt about 1600 BCE, but were kept in very small numbers in the Nile delta and along the river. By the 8 th century BCE, horse breeding was done for the Egyptians and the Assyrians in southern Nubia, where it was possible to have pastures even some distance from the Nile floodplain. The success of the horse in the Near East and Egypt relied on the development of the proto-Arabian breed, which possessed a variety of physical adaptations that allowed it to live and work in extremely hot and arid environments. The earliest evidence for this is around the time of Egyptian pharaoh Thutmose III (1479-1425 BCE), and his source was probably in the region of Syria-Palestine. This may be the turning point after which it is reasonable to look for the horse moving into the Arabian Peninsula.
4. Basis for Contrarian View
From OP's comment asking why I took the "contrarian view":
I guess what I'm saying is if you can quote actual recent scholarship that establishes the contrarian view ...
Short answer: Instead of just more recent scholarship, I am more inclined to believe Postgate is correct. Dalley not so much. The reason for this is explained below.
Kingdom of Kush, began in 8th century BCE, were constantly at war with Neo-Assyria. So, the real question is would horses be sold to the enemy. Some historians believe so, others considered this unlikely (contrarian view).
4.1 Dalley vs Postgate in The Horses of Kush (1997)
First, as OP explained, the key document is Heidorn's "The Horses of Kush" (1997) and, for me, it was a choice between two academics who disagreed on the provenance of Kushite horses, namely Dalley & Postgate.
- Dalley - The author referenced Dalley (Stephanie Dalley, a
British scholar) as the source for this idea that Neo-Assyria was
buying horses from the Kushites (see OP's quote in last paragraph).
- Postgate - He is the competing academic who did not agree with
Dalley (also referenced by Heidon), and it was Nicholas Postgate, also a British scholar. In addition, he is an Assyriologist.
- In 2007, Postgate wrote more on Neo-Assyria's reliance on horses from Nubia (see below), but as for Dalley's "Kushite" horses, I could not find anything else on this topic from Dalley.
4.2 The Land Of Assur & The Yoke Of Assur (2007) by Postgate
The full title of the book is "The Land Of Assur & The Yoke Of Assur: Studies on Assyria, 1971 to 2005" (Oxbow Books, Oxford, 2007).
In chapter 6, "The economic structure of the Assyrian Empire", Postgate made the argument that Nubian horses were provisioned (not bought) from Nubia, especially during the reign of Esarhaddon (681 – 669 BCE), pp. 217-8:
Problems of supply: Alongside the general dependence of the cities on the country, we should put some specific areas in which the central government relied on supply from a different part of the empire. Apart from corn, the staple requirements of the army were for men, animals, and straw. The provision of men and straw seems to have proceeded smoothly as long as the empire remained intact, but it is clear that the supply of animals, and especially of horses, was a constant anxiety to the kings of Assyria.
Texts from the reign of Esarhaddon show that the horses were classed in two broad categories, riding (pethallu, cavalry) and draft horses (sa niri, of the yoke), and the latter type includes horses of the Kusaean and Mesaean breeds. There is no certainty of the identity of the places Kusu and Mesu from which these names derive, but the most plausible candidates are Cush (or the Sudan) and an area of north-western lran. Horses were certainly bred in Assyria as well, and elsewhere, but nevertheless the diminishing control over Egypt and the far north-eastern parts of the empire during the 7th century may well have led to a shortage of horses and thus have affected the whole country's military potential.
4.3 Nubian Horses Levied or Seized
A 2008 University College Dublin thesis, Horse Breeds and Breeding in the Greco-persian World: 1st and 2nd Millennium BC (2014), published unabridged -- after some explanation of how many Kusaean and Mesaean horses Assyria needed, the author stated these horses were "levied" or "seized" (not bought). In p.172 (emphasis mine):
The total number of horses levied in the the three months of that came to 2,627. Many more tablets relating to horse levies no doubt existed but have subsequently been lost. ... Constant warfare led to large numbers of horses being lost and so we find many references to continual campaigns where many thousands were seized.
To end, I am certainly not saying the proof is rock-solid, that Nubians (Kushite kingdom) did not trade in horses during the 25th Dynasty. But the circumstantial evidence (war with Neo-Assyrians, prestige economy, etc.), makes it seem unlikely. That's all.
Having said this, the most likely person with knowledge of the Nubian horse "industry" is Sandra Olsen, curator-in-charge at the Biodiversity Institute and Natural History Museum at the University of Kansas. She was consulted by the principal investigators of the "Tombos horse" and she is editor of a new (in press) British Archaeological Report, Equids in the Ancient Near East, Egypt and Arabia, BAR, Oxford. (I do not have access to this report)
Note to readers: The comments below does not make sense as I've deleted mine and incorporated them into the answer.