I'm curious as to how crucial a node Rome was to long distance Mediterranean trade in the early imperial era, from the Augustan to the Severan dynasty.
I know that for late Roman trade, the twin spines of Carthage-Rome and Alexandria-Constantinople were absolutely essential, but that east-west trade was at an absolute minimum. I believe this is a reflection of the increasing pattern of regionalism that began with the 3rd century crisis onward to its conclusion in the medieval successor states to the empire.
But if the early imperial days were not this way, that would presuppose (as I know some historians have proposed) that the broader early imperial Roman economy involved more exchange activity. Thus there would be more comparative advantage in production from farmers on up, which raises the possibility for beyond-regional distribution of such produced goods.
But how far a range would these beyond-regional distributions go? Alexandria was obviously as much of a hotbed in the early imperial era, but we don't have Constantinople as an alternate center, thus I'm sure their trade was primarily with Rome.
But was Rome such a necessary spoke that most trade would still need to feed through there on its way elsewhere? Were regional economies strong enough outside of Rome to provide a competitive market for products? Or did the city of Rome's consumption define all trade?
This is a very interesting question, and I hope it sparks an interesting discussion. I'll weigh in here as I am writing my dissertation on the traffic of grain in the medieval period and have read a lot about the Roman taxation/redistribution system in preparation. Before I give my two cents, however, I want to ask a couple of questions.
First, what do you mean by "beyond-regional" distribution of produced goods? Could you give an example of a hypothetical exchange of goods that fits your conception?
I think your question gets at the heart of the nature of Mediterranean trade, and I want to make sure I'm following your train of thought correctly.
There are a number of scholars that have written about this problem. M. Rostovtzeff's Social and Economic History of the Roman Empire would be the place to start if you haven't looked at it already.
Regarding your particular question, I think that we need to look past the central redistributive axis to see elements of "beyond-regional," or inter-territorial(?) trade. Although the Emperor and his government in Rome represent the single largest apparatus for the redistribution of goods, other regions of the empire must have served as financial hubs. We can see evidence of this throughout Roman history.
During the late republic the government had been informally importing grain through legislation the legislation of what have come to be termed populares politicians since the Gracchi (late second century). Rome did not, however, represent the sum of all Mediterranean commercial exchange. Caesar banked with a phoenician based in Gades (Cadiz). The Phoenicians, particularly the Carthaginians, had been developing eastern Iberia into an important commercial hub throughout for hundreds of years before the Punic wars. Cadiz was founded in the 12th century BCE. The economic development of the Iberian peninsula wouldn't have devolved into some sort of subsidiary of Rome as the empire progressed. The region would have certainly exported olives and wine to Rome, but it was also a major exporter of metals, which were used throughout the Mediterranean.
Other pieces of evidence for the economic development of the western Mediterranean basin can be found in the records of the late republic. Massallia (Marseille) was founded almost as early as Rome (around 600 BCE). By the late republic, a number of Roman senators lived in exile there.
We also need to consider the eastern Mediterranean. This region must have supported a large amount of inter-territorial and extra-imperial trade before the foundation of Constantinople. Asia province was the wealthiest territory of the empire from the point that it was incorporated through much of the history of the principate. The cities of Ionia were important throughout much of classical and Hellenistic history. For evidence we need only look at the seven wonders of the Ancient World. The Mausoleum of Halicarnassus and the Temple of Dionysius at Ephesus were both built in Ionia. The region was also part of the inter-imperial trade of the Hellenistic and Imperial periods. Greek speaking polities extended from the Aegean through what is now northern Pakistan during the Hellenistic period. These polities traded with the Maurya Empire (322-185 BCE) and later the Gupta Empire (320-550 CE) and the Chinese Empire of the Han Dynasty (206 BCE-220 CE). I recently read about an archaeological find that included both silks from China and a wool tapestry from the Roman Empire here, https://www.worldcat.org/title/silk-road-in-world-history/oclc/646816188&referer=brief_results. Furthermore, roman glass was an in demand product in China. All of this exchange would have taken place outside of the redistributive axes of the Roman government. Furthermore, the purple dye of the old Phoenician cities continued to be produced, and not all of that was bought and sold by an apparatus of the Roman Emperor.
Constantine had a number of reasons to found a new capital. Christianity was more prevalent in the east, he had recently won a civil war and a new capital founded in his name would have been a dynamic move, but perhaps most importantly, the east was simply wealthier. The foundation of Constantinople represented the creation of an imperial hub that was located at a crucially important location that facilitated trade between the Black Sea/Romania/Balkans and the Aegean. Rather than creating a new redistributive axis from scratch, Constantine was responding to a commercial pattern that had developed for hundreds of years before the foundation of the new capital and indeed the rise of Rome.
This suggestion proposed here is supported by archaeology. The largest number of premodern shipwrecks have been dated to the principate. Not all of these were found along the route to the capital. This graph was shown to me by a friend, and I am not familiar with the dating system used to come up with this position. Regardless, I think that we need to assume that the redistributive apparatus of the Roman tax system represents a significant, but not the entirety of Roman period commerce.
I can't really give an extensive answer on this now, but broadly speaking, probably a great deal. We can't give numbers, but the material remains of trade show this, such as red African slip ware pottery, which turns up everywhere (including in the late imperial period). There is also a fairly well documented "northern Mediterranean" network including northern Spain, northern France, Britain and the military regions of Germany. More compelling evidence may be the epigraphy, and merchants from eg Asia show up in Lyon, and north Africans in Austria.
Rome was the largest market of consumption and so was naturally central, and we should also not forget that Italy takes up an awful lot of space in the Mediterranean. But this wasn't to the exclusion of other markets.