So hows this for a complcated suggestion?

All Legion troops (or faction focus troops like horse archers for parthia, phalanx for greece ect) cannot be replenished anywhere, and must be re-trained in a building suitable to recruit them, ala R1.

All Auxillia or standard troops (like almost all factions have two barracks chains this could be tied to this happily) and mercenaries replenish according to infrastructure and other effects, such as food surplus, with a relatively small nerf being applied if no troop-producing buildings are nearby.
With the exception for... Rome... they have 2 buildings, one of melee and horses and one for javelins, slingers, archers and mounted ranged units (slightly different for barbarians). You end up with auto replenishing all the ranged stuff and having to send back all the melee stuff. That either makes factions like Rome pretty op or you just run around with ranged stuff all the time and everywhere. I guess a stack of peltasts and skirmishing cav can still be quite hard to deal with.