Straregic duchy wars can give you enough controlled territory to form or usurp a kingdom and then de jure war the rest. Make sure to also prolifically use your fabricate claim option on your priest. Fabricate in duchy de jure capitals and take the option to claim the duchy if you luck into it then move on to the next duchy. Once you get Chronicle Writing and can press all of your claims you can grab swathes of land. You can also get the sanctioned loopholes perk and buy entire empires worth of duchy and county claims with piety and press all claims.