WI: The fates of the Visigoths and Byzantines were reversed?

What if the Visigothic Empire and Byzantine Empire had reversed fates from the 700s on?

The Visigoths repel the Umayyad invasion, surviving for centuries, sometimes expanding and sometimes declining until an ultimate fall. Afterward an Ottoman-like empire (but Berber) takes over and rules al-Andalus.

On the other hand, the Byzantines are taken out in one war by the Umayyads, but successor states in the mountains of Greece launch a Reconquista of Greece, Thrace, and Anatolia. This takes centuries, but ultimately leads to several stable kingdoms. These kingdoms unite into a "Spain" (Thrace and Anatolia) and a "Portugal" (Greece).
 
It's a scenario I've thought about.

My vague idea was a fall of Constantinople in the 670s, followed by an Arab conquest that takes in Thrace and the approximate territory of modern Greece, but fails to penetrate much further due to internal issues in the Caliphate. The conquest of Africa fails to penetrate any further than Tripolitania. The old African Exarchate slowly evolves into an independent Christian state that manages to fend off the Berbers, take in the remaining imperial possessions in the Western Mediterranean, including Sicily and southern Italy. All trade from the Islamic world into the old WRE now must flow through Carthage, and the city flourishes, even mounting expansionary phases of conquest that see the Arab emirs of Crete and Cyrene vassalised, and raids even into Egypt.

Meanwhile, the northern and western Balkans become a war-zone, as generations of Arab commanders lead their troops west out of Constantinople to do battle with the heathen Slavs and Bulgars. In an unexpected coup, Christian Frankish merchants manage in the middle of the eighth century to achieve dominance over the Slavs in what is now Serbia, and form a kingdom that claims continuity with the old Eastern Roman Empire, helped by the still surviving Balkan romance communities along the Danube. This alt-Serbia manages to achieve a period of temporary dominance over nearby Bulgaria too, forcing the Bulgars to convert to Christianity and, bit by bit, the Arabs are dislodged from the Balkans, a process helped by the conflict between Arabs and Islamised Greeks in the region.

While this is ongoing, Islamic Egypt achieves a degree of success in conquering and converting the Sudan, but is itself overcome by Islamised Sub-Saharan Africans, who first take over Egypt, and then break the army of the African Exarch, swiftly overruning much of the kingdom besides Carthage itself and a few coastal holdouts. They eventually come to overrun the Visgothic Kingdom of Spain too in allegiance with the Berbers. The result is a great southern Mediterranean Sudanese Caliphate, stretching from the Pyrenees to the Nile. By the sixteenth century, its greatest rival is the restored Christian kingdom of the Balkans and Anatolia, which has, in allegiance with the Armenians, forced its way down through Mesopotamia to gain access to the Persian Gulf and has established great naval power and wealth via Indian Ocean trade.

How's that as an inverse reality?
 
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