Elective Dukes.
When you are trying to manage vassals, you have three general interests: taxes, troublesome personalities, and preventing the rise of a power base. None of these are particularly hard- you can always build up your personal domain for 'enough' gold and dread-suppress the masses- but you'd still enjoy more taxes and less trouble in the near term and long. Putting feudal elective on every duchy supports this, especially as you get higher crown authority to stop internal wars.
1 - Taxes
In terms of taxes, the biggest issues come with Counts. Counts are- in any given form- the worst tax payers. This is because rulers typically only pay 'full' taxes to their direct de jure liege. Non-de jure vassals have a -50% taxes, but even Counts-to-Kings (or Emperors) are -25%. In adddition, when you put Counts under dukes for Vassal Limit purposes, you run into the tithe cuts. If a count's county produces 10 gold, but each level of taxation means there is a shorter and shorter share that goes up the chain. 15% of that 10 is taxes to the Duke, 15% of that 1.5 is taxed to the King, and so on. This gets worse when you start figuring in frequent control drops. Control loss from wars, occupations, and such proportionally reduce taxes. 25 control is -75% taxes. Since counties are cursed by partition, and thus generate claims, and thus claim wars, counts pay less taxes in general.
The main exception here is Dukes. Dukes pay full taxes to their de jure Emperors as well as Kings. Moreover, Dukes pay 100% of the taxes of the counties they personally hold. The best way to tax a county within a duchy is for that county to be personally held by the de jure duke.
Elective starts to shape this because an elective duchy that can mitigate partition. When this applies to the AI and not just the player, this means the elected duke will have their domain limit composed of those de jure counties. This mitigates the need for counts- and those partition civil wars- and so increases the tax flow potential.
2 - Personalities
AI trouble is driven by personalities as well as opinion, and elective as a modest mitigating influence on both. Elective votes have a bias for characters with high diplomacy scores, higher opinion modifiers from things like religious virtues, and few personal scandals / feuds with others that might spark sins. These are, at least in the Christian world, all aligned with the dominant Christian virtues, especially the likes of Compassionate and Honest. While there are not guarantees, over time / generations there is a bias towards elective duchies electing virtuous dukes. This is especially true if you- the higher ruler- take custody of the children for a few years to share your personality of own virtues. Once the AI has a line of virtues, AI rulers who guardians of their children will apply the trait bias for transfering their virtues, which can create a virtuous cycle of virtuous traits.
These traits don't just win the elections, but are the AI personalities you want vassals to have. They are vassals who are typically more social (holding feasts), less greedy (fewer wars), less vengeful (murder plots), and so on. This in turn leads to AI personalities more likely to spend on activities (and thus mitigating stress / giving you free invitations), upgrading their domain, and so on. These also- because of virtuous religious opinion buffs- make the people more popular, and more likely to be voted over an Ambitious-Sadistic monster.
These election-winning traits, in turn, bias AI personalities towards the Courtly AI faction. Courtly is great because it is the easiest faction to please / increase opinion modifiers of / least bothered by crown authority increases. The Courtly personality traits often come with diplomacy and opinion bonuses for eachother, and get pleased by things like noble marriages and feasts. However, what makes Courtly really good is the opinion malus mitigation for higher crown authorities. High crown authority increases taxes and power at the cost of opinion. However, while max authority has a -40 opinion for most factions, it is -15 for Courtly faction.
Elective starts to bias your dukes AI personalities towards the the sort of AI vassals you want, who are less troublesome and more likely to happily pay taxes.
3 - Power Base
AI trouble *over time* comes from the AI doing what a player does: devouring the Kingdom from within by subjugating the other vassals until the Duke dominates multiple duchies, grows fat with their gold (for troublesome mercenaries or things) and levies (for reaching the revolt threshold). Even if the AI isn't inclined to act poorly, those are still taxes of courts going to that Duke, rather than from another duke directly to you. Remember- by structure your ideal is the Duke holding the county directly, and paying taxes to you directly.
Elective supports this by (peacefully) breaking apart mega-duke power blocks, except in the more favorable cases. When a Duke has multiple elective duchies, they also face the point that every count in the duchy gets a vote. As a result, Dukes easily get outvoted in the duchies where they aren't holding the counties. This- usually- means either that local counts start to vote for themselves, and thus become the next dukes, OR the duke is so virtuous that they win the elections regardless, but their personality keeps them unthreatening by nature.
This power base division gets stronger at higher crown authorities. Not only do those provide more taxes, at the highest level they also block internal wars. Once the Duchies can not longer declare internal wars, elective duchies become exceptionally resilient ways to break apart power bases, as claimants can declare for those wars... and even if they do assassinations, there is no guarantee they will be voted next. And since the murders may be exposed, are less likely to win and give you a troublesome, murderous, vassal.
Start combining these, and elective duchies start adding up to a series of supporting synergies. There are no promises/guarantees here- nothing that promises that only the virtuous will raise- but collectively these start mitigating issues in ways that don't require you directly suppressing the revolt.
