The 8 civs at release were somewhat focused on the high-late middle ages, especially European ones, from their shield and armor designs. For example, even the early knights and man-at-arms have heater shields which were a 13th century thing, and close helmets which are even later. Upgrades only change their textures from “leather” to steel to gold.
The intention may be consistency. However, (like in every previous AOE), more updates to the game introduces civs that are focused on an entirely different timeframe, so the consistency no longer holds anyway.
For example, the Byzantines is almost solely centered on early middle ages. Limitanei was a late Roman Empire thing (when the Western Empire was still there). Units start with round shields and upgrade into kite shields, something earlier than most other civs’ visuals in the game.
On the other end, there has been also firearms as advanced as arquebus when most other civs still have the very primitive hand cannons. Some like Janissary can be trained earlier. Meanwhile Japanese have sengoku ashigaru using hand cannons which is a combination that never happened.
Because of these I think it would be a good idea to extend the timeframe of unit visuals of the vanilla civs beyond high/late middle ages, into both early middle ages and renaissance.
More specifically:
Dark: early middle ages featuring round shields and mail armor.
Feudal: 10-12th century with Norman helmets and/or greathelms, and kite shields.
Castle and Imperial: what they look like now, but with a late game technology that changes the weapon of hand cannoneers to arquebus.
Some of these assets are already in the campaigns