I studied this a ton for The Raptor of Spain and wrote a very long post explaining the dynamics of Al-Andalus. And then AH.com ate the post when I tried to submit it.
Sorry. In a nutshell: there were religious, ethnic and cultural differences that combined with the pressure of the Christians and that rulers only had powerbases in their particular slice of the religious, ethnic and cultural makeup of Al-Andalus meant that to actually rule you had to forcibly hold down everyone else with your particular group. This is why after Abd ar-Rahman III and Al-Manzor Al-Andalus disintegrated. They had held everyone down for 80 years but it was only force that covered the differences and the backlash created the Taifas.
Maybe if A a-R conquers the Christian states, abandons any ambitions to regain the Caliphate early on and aggressively pushes the Hanifa doctrine of Dar-al-Islam.