My suggestion.
Income
Generating more income can fix the poverty problem.
Currently a fief's income unable to feed alot of troops or garrison. If we player is rich enough, can't we just store food/ money to help out our own fief? Or even our own village. Or rebuild those burned down villages, build more houses to attract more refugees to come & stay. It can increase loyalty & prosperity.
If a fief can generate enough income, those faction leader(rich folks) with fief able to pay enough "wage" to those lord without a fief so that they can recruit enough men to continue fight. Wages amount depends on clan tier maybe?
Maybe with this, those weaker kingdom won't be eliminate so easily? I can think of many ways to upgrade a fief's income you can implement. Or maybe implement more ways to earn money for those clan to increase their income, not just only depend on fief's tax & caravan. Maybe pillaging get denars (amount depends on the prosperity)
& also make those kingdom who captures too many land will automatic become the common enemy of all other kingdom. Instead of all kingdom just targeting us (the player) because of the conspiracy.
Rebellion
If we have more garrison in the fief, it can prevent any rebellion to happen
If rebellion successfully secure the town, the town should restore back to high loyalty. & 'rebellion clan' remains in the fief until any other lord take it down.
New born clan
If rebellions are remain in the fief, maybe new born clan can be in the game depends on number or lords in the game running around. If there is lesser lord in the map, new born clan will appear. (to keep the balance. If poverty is fixed)
Player perspective
In late game, player has too much money/loot & no where to spend, even those fief only has 20-50k, can't even able to sell off all our loots. Maybe can add more stuff for player to spend? Example like upgrading fief can be expensive maybe? Instead of just "buying noob clan with overprice"