I had in landed title the restriction NOT = { tier = king } for Kingdom of aquitaine.
The main reason is that after an occitan king of france create it 3 time in a raw in 3 diffirent game i start, breaking the immersion (why should an king create another crown weakening the one he hold allready, when succesion law is primogeniture?) and probably because of games mechanics, switch primary title time to time...
That work pretty well (in my test game the Kingdom was create by the IA after the Duc of Aquitaine manage to gain independance) (I play PB+SWHM+Viet Traits and Events+no ahistorical empire)
Main reason are are historical plausabilty and its the SWHM file i edit, but since your focus is balance,maybe you could consider this to improved the France capabilty of surviving in 1066 start.
My knowledge for modding and i have not so much time to play so my personal gameversions have very limited scale (for instance i also disable the kingdom of Britanny for personal reasons) as your work is just awesome anyway.

But i like to know if there is a simple way to force the the succession for different titles (for exemple give up the kingdom of Jerusalem when hold by France or England King)
Regards.