Theres a term for non-gold units: trash units
Currently there are not much trash units in game, basic tool age archer, slinger, axemen and with bronze age chariots. Chariots are the better performing trash units, so if you are in late game and you want non-gold units to swarm your enemy chariots are the only ones to allow that.
I think thats the main problem, chariots being fairly good, and only counterable by other chariots or gold costing units, like you guys said camels and stuff.
Trash units should be counterable by other trash units, both sides of the battle should have access to pull this off. In AOE 2, this exists, you have units like halbediers aswell, so you can counter a possible trash unit swarm.
I think AOE:DE needs one more unit, preferably some sort of spear/halberd wielding non-gold barracks unit.
My suggestion: He should be fairly weaker than a gold-costing barracks unit, for example a swordsman. Maybe something like 40 food 35 wood? He has damage multiplier working like a slinger, low damage(2,3 or 4 at most) but dealing high x2 damage to cavalry units. This may even provide some sort of defense againts Yamato bronze cavalry spam.
Btw, slingers fall short in late game, cant really beat up chariots anymore. Whatsup with that? Apperantly they dont get upgrades like rest of the armies do. Just why? Wouldnt it be good to see slingers(a trash unit just like chariot) to have its own use in bronze & iron age?