Having uneven numbers per architecture set is not really an issue.
Civilisations having completely wrong architect is.
Assyrians is right next to Babylon, their architecture should be the same.
Now that the Mesopotamian architecture includes gates with lamassus on them, for Assyrians I feel like this is even worse than it was in AoE1. Personally, I would balance things a bit by giving Hittytes the Egyptian architecture – neither set is accurate in their case anyway.
“Indians” had their own architecture for ages and nobody said anything. If we want more use of the Egyptian set, then the Nubians and Numidians should be added, not giving it to thoroughly incorrect civilisations.
Honestly this is even worse with the Sumerians, as they now have a campaign which not only describes it being set in Mesopotamia…but also uses Lamassus as the icon for each level!
If an Israelite/Hebrew civ is ever added, I could see them using the Egyptian style, as it is the closest thing to generic Middle Eastern architecture, and they also have a historic association with the Egyptians if you believe the Bible (though this is of course disputed nowadays).