Thanks for the help! I took my time, explored the world, helped out my allies even when there wasn't much point, and eventually I did it - after 28 years I've established the kingdom of Ireland
Elective seems pretty great, even with most of my titles held by family members. The maluses seem pretty minor so far (after the initial period of rage).
My current ruler actually only had one son, an absolute moron with awful stats who murdered a bishop and took an oath of celibacy. Fortunately my brother was the promiscuous party animal, so I have plenty of nephews.
I really want the oldest non-bishop nephew to be my heir. He's the badass administrator married to the Petty Queen of Gwynedd. Right now he's Mayor of Wexford, my Steward, "King" of Gwynedd (duke), and my heir (with zero votes, somehow). His governing stewardship is 48 (19+10+19). He's also just a nice guy and hasn't given me any trouble.
I can't seem to actually nominate him as my heir, though... oh. Nevermind, I reread the tooltip for Agnatic-Cognatic Elective and figured it out. Nominees have to be children, siblings or dukes in my case. No problem, I actually wanted to give him a petty kingdom! What's the worst that could happen eh?
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Well I don't know if this is bad but, doing that made "great Houses" form in the petty kingdom. Oh jeez. I just promoted a burgher to a ducal title, didn't I? Yeah, "Grand Mayor", and I still can't nominate him as my heir.
If I can live with this -30 for "wrong government", will this guy make me and my heir loads o'money? He has Dublin and a port...