I'm playing Nubia emperor for the first time now. I recommend barracks. You've got an already-large xp bonus that lasts all game. Barracks plays to your strength. Double-shot xbows are blistering. Two shots can take out a courser. Put off upgrading your pitatis until you have to--that extra movement is big.
Yes, Nubia is strong building districts. But I wouldn't spam them just yet, until the timing is right. Later you can make a monumentality era dedication, which can propel you to a Golden/Heroic age. You can always start them early, but you get the era bonus when you finish.
P.s. I'm pretty sure nubian pyramids are not stackable, as far as +20% district building bonus.