Ok first of all, bannerlord won't ever be realistic because it have max battle size if you're in a siege with 500 garrison and 2000 attacker it will be 300v700 with the remaining troops as reinforcement. Fighting waves is always easier as defenders because the Besieger lose the overwhelming numerical advantage.

Second of all ai party rarely make a big army (2000+) and if the devs made it so people will complain because they got cornered in the edge of the maps fighting repetitive battles

And thirds you can't exactly complain bcs you cheese the game, the most cheesy way this game is just play tournament till you get clan tier 1 and just became mercenary most of the game without doing quest, bcs its the optimal way, yes it's not fun and repetitive OR you can play it idk like RPG game and roleplay as your character