Ok I realised there is a pretty strange bug which allows you to rig the succession system for 100 years. I was playing Castille and I owned all of the land in de jure Kingdom of Castille with elective succession by personal preference to keep vassals happy and sometimes the eldest son isn't preferable. I annexed and destroyed the kingdom titles of every Kingdom I conquered apart from Castille (entire Iberian peninsula, Morocco and Mali) to incorporate them as the Kingdom of Castille after 100 years. (I did this because Empires always end up fighting over shit and breaking off since vassals can have tons of land as kings, and kings always want independence). Anyways I realised that I was the only elector in the entire country and whoever I elected would always be the successor and everybody was fine with this. I found out that its because there are no dukes in the Kingdom of Castille and destroying Kingdom titles revokes the elective rights of anybody in that Kingdom until it incorporates.
This means for 100 years until one of the Kingdoms incorporates and there will actually be other dukes than myself in the de jure Kingdom of Castille, I am the sole elector in the entire Kingdom.
This means for 100 years until one of the Kingdoms incorporates and there will actually be other dukes than myself in the de jure Kingdom of Castille, I am the sole elector in the entire Kingdom.