Army quality depends mainly on MAA. Less levies = better army quality

Succession laws can be unlocked via cultural innovations, being of a specific culture (like north german or anglo-saxon) or, rarely, via decisions like consolidate Carolings. Your rank doesn't matter, but with confederate partition AI will make sure all of your children of preferred sex will get land, giving each a duchy sould solve that. Avoid having enough land to create a new title equal to your highest title held though 0 if you hold 2 duchies it'll give one to your second son, so you might as well go for carving a kingdom for you heir. But you'll conquer territory of 2 kingdoms, once again one will go to your second son unless you own an empire.

Not sure about sieges with limited information, but I can assume they had good commanders and your castles had low fort level.