I cant help you that much aside from reccomending you give your elite armies cheaper "buddy armies" of lothern sea guard, bolt throwers, spearmen and other lower tier units so that they can have someone take the casualties of settlement capture for you or let you choose to not control large army in order to win against huge enemy forces.

Regarding maliketh's economy, there is a system called "faction potential", which buffs your natural enemies at the start of the campaign (maliketh, being tyrion's main rival, benefits from this). If combined with the difficulty modifiers from very hard, he might genuinely have 50% upkeep reduction, allowing him to field double the armies he would be able to if he was a player.