From Early Medieval Spain by Roger Collins:
Unfortunately the question of the Berber contribution to the culture of Al-Andalus has received little attention. It is often assumed that they were thoroughly Islamicised by the time of their entry into Spain in the early eighth century. However, the Arab conquest of North Africa was hardly complete by that point. What is more, evidence of the continued use of Latin there can be found for as late as the tenth century, and substantial Christian communities were still in existence in Africa in the eleventh. It is thus highly improbable, particularly in view of the general indications of slow conversion to Islam on the part of the Arabs’ subject populations, that large numbers of Berbers were Muslims in 711, nor in view of their role would they have been required to be. A reference in Al-Makkarī to the religious practices of Berber rebels who besieged Mérida in 742 makes it clear that they were not Muslims.
Without in any way being expert on the topic of the expansion of Islam this runs counter to what I thought. Probably the biggest first-pass lesson I get out of reading history is that the most common mental error in thinking about history is to project today backwards onto yesterday. I doubt I am alone in this.