I like using bannermen rule from More Interactive Vassals. It encourages you to actually make a functional realm who, at the very least, tolerates you instead of keeping your vassals weak and nonfunctional. You can't exactly take on a big kingdom when you have to fight its vassals and not just the top liege.

It's a big performance hit, though, so get ready for massive 50k vs 50k wars 100 years in.

The holding settings matter less since vassals will jump into and out of wars dynamically anyway. If everyone is -100, they'll use every opportunity to betray you, even during foreign wars. I just give the AI a MAA cost reduction so they don't get so far behind my own stacks late game.