With careful diplomacy and a bit of patience (don't conquer too much too quickly) you should be able to extend the realm divide trigger in vanilla to 20 provinces. The diplomacy mod I've used (from steam) pushes that out to 40 provinces - which, to be honest, is too high and makes things a bit too easy as a 40 province faction is basically in total control of the map with excessive income and the subsequent unbeatable military resources. Also keep in mind that Provinces made vassals don't count towards the realm divide trigger and are very useful as willing trade partners. I'm going to ditch the diplomacy mod in my next campaign. I actually miss a meaningful and dangerous Realm divide
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