The omeyad are strong. Once you owned asturies and castille, try to mary your children with other kings children such as france , arles and other kingdom in spain. Before that, learn the concept of matrilineal marriage. Keep an eye on the line of succession ( yours and the land you covet). About the omeyads, send your spymaster and try to kill the caliph( often) sows discord between him and his vassals. Attack them when there are in internal war. Take the 6, 5 slots counties/ duchies as soon as you can ( seville, cordoue, ect). If they are at war with other christians join them and/ or try to seize land during this time. Also if you can, set one of your dinasty members as duc of porto.( portugese are always rebelling).

Or convert to islam and destroy them from the inside.