Meat shield or slave workers unit, have i understood correctly?
IMHO the boyars should be more armored version of lithuanian Leiti with ability to switch to ranged weapon. Also I can propose much more Asian civs (especially related or closely contacted with nomads of Great Steppe) have this type of unit (mounted lancer switching to archer), armoured version could replace vanilla knight, however i’m not sure in which way the light version could replace existing steppe lancers, mounted archers and/or hussars.