I think it’s far cleaner to just give them a bonus when they conquer a city equal to the max hp of the city (before buildings are lost). That encourages them to actual conquer (or raze as desired). And it’s far less micro intensive (which the Huns already have a lot of clicks built in due to reliance on a skirmisher UU).
", which isn't technically gated by conquest, but is locked behind nukes. Not much better if you can only get around using it as an on-conquest bonus via nuclear holocaust.