When you get bigger armies is also the time where you should get fiefs.
A good use of a fief is to store troops (in the garrison) for replenishment purpose (plus they train a bit with appropriate building). It'll greatly lower the down time of your army after taking a beating.
Of course you have to keep the garrison with the troops you want, can be a problem if your fief has a different culture. Not a perfect solution but definitely helps.
+1 on ranged troops, use loads of them. Melee are arrows/bolts magnets, cav for disruption and ranged for the actual killing. 30% melee, 10% cav and 60% range is my usual army comp for battania/vlandia/empire.
Also avoid, if you can (if the AI can win without you), big battles (2000+ fighters). The way reinforcement works in bannerlord, you're bound to take heavy casualties at some point.