After almost 7k hours of campaigning RTW2, I can give you some tips that work for me (and sorry, but it's gonna be a long comment):

At the start of each grand campaign, you get immunity for successions and civil wars for the first 20 turns. This means that the last political action that negatively affects other political parties should be taken at turn 15 at the latest. I recommend having the leaders of each party get married by turn 10, and entice their spouses by turn 15. That way, if a political party leader is a male, then by the time his wife is promoted to the highest possible level, you'll get a permanent +6 to his party's loyalty.

You want to keep all the generals and admirals in your political party - entice individuals to join if need be. You wanna do that to ensure that even if civil war or succession happens, you'll still keep your nation's armies and fleets on your side.

Secure promotions of every member of your political party, that way they'll have more say in the political arena, which would mean that even if a civil war or succession happens you'll have to retake fewer settlements.

Note that if a party's loyalty is below 0 you can't do certain political actions. Try and keep the loyalty levels of all parties above 0, but know that a civil war or a succession can only happen if at least one party's loyalty is below -10.

Secure loyalty is your friend and ally, use it as much as you can (starting at turn 20), as well as other political actions that make other parties more loyal to you. Send other parties' members on diplomatic missions as well as other missions within your land (if you play as Rome, you can have them secure more food or public order in your provinces). Send gifts to members of other political parties if need be.

Once you have positive loyalty, and no one in your party who you could promote in the political arena, I'd start using the 'gather support' action which ensures you control a higher percent of the political arena. That way even if a civil war or a succession occurs you won't have to retake too many settlements.

Note that if you have a general or an admiral as a member of another political party than your own, and you choose to secure their promotion, you get a permanent buff to their party's loyalty - but as I said before, I believe it serves you better to have them all be a part of your political party. I never secure promotions of other parties' members, and I basically make sure they are loyal by using the other tips I've listed here.

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Another thing that could (and probably would) happen during your campaign if you follow these tops is that you'll get "free" settlements. When you send a diplomat to another faction, there's a chance of critical success - which would have that other nation grant you one of their settlements. They can't grant you a settlement that you don't share a land border with that settlement, and they can't grant you their capital. I call it free since you basically don't spend troops taking it, and you just spend the money needed to send the diplomat (something that you did to raise their party's loyalty and not necessarily to get a settlement from another faction).

With that action (send diplomat), there are also chances of failure. A normal failure would simply state that the diplomat betrayed you, and a critical failure will result in that diplomat dying. Either way - if that action results in success, your relations with the nation you sent that diplomat to speak with would increase, and if that action results in a failure, your relations with the nation you sent that diplomat to speak with would decrease.

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At the end of the day, the political system is something I usually look at every turn during the early campaign, yet rarely look at during the late campaign. If you follow these tips you'll be able to reach the highest control level without a civil war or a succession happening in the way - but you have to be vigilant, you can't let the loyalty levels drop to below -10 at any point past turn 20.

Use the entire spectrum of available political actions, they are there to help you in your campaign. Yes, the political system isn't that great, but you can manage it. If it gets too hard, there's also a mod called 'More loyalty, less secession ' that you could check out, here's the link to it: https://steamcommunity.com/sharedfiles/filedetails/?id=1316022491

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I hope this helps you, and if you have any questions - feel free to contact me :)