I don't think rams are OP. Once players figure out that swords and melee destroy them, they are pretty harmless. Right now because of the strength of capturing, you oftentimes don't need siege at all.
I don't see the point of requiring them to be garrisoned to move, as one could argue the 3 population cost is three dudes that come "pre-installed" inside the ram to push it.
They should cost 0 pop, only wood and maybe metal. Any population cost would come from soldiers garrisoned inside. (This would probably make rams garrisonable on ships without taking up space though).