I certainly hope you're right, but I doubt future Egyptians (and Sicilians, Carthagians, etc.) will be happy serving as a economic satellite of Rome. Basileus has already stated that current economic relation suits Rome just fine, but Egyptians are less pleased that they are being used as a captive market and resource (grain) extraction colony.Fair, but this isn't OTL. Cultural development is extremely different and nationalism is still an uncertain beast at least two centuries away. "Full independent" is likely never going to happen based on the style of empire B444 has led towards. High autonomy sure but a federal empire would absolutely have a unified foreign policy.
Once industrialisation comes Despotates will inevitably want to develop their own industrial base. If they're integral Roman territories that's fine, more income to tax and increase in warmaking potential. If they are semi-independent they are competition.
Therefore, in my opinion federal empire in USA/current German mold - great, federal empire in interwar Commonwealth mold - less great.