Whoa.
First we need to establish how the Ummayads have such overwhelming successes. While Arab naval victories and eventual dominance of the Mediterranean occurred in OTL, and while Arab armies were more than capable of raiding deep into Anatolia, and while the Ummayads were able to besiege Constantinople, even with everything going well for them, I struggle to imagine how we make it as far as you're proposing. Even if Constantinople falls after a lengthy siege and Arabs are capable of raiding across Greece, how do we end up with a permanent conquest of places like Illyria? It seems unlikely, especially given that an Arab conquest of Constantinople is more likely to simply open up the floodgates for Slavic and Bulgar invaders.
At some point there will need to be a period of consolidation. Constantinople is far from the Syrian heartlands of the Caliphate - Italy even moreso. The conquest of Spain by Arab and Berber warriors was separate in many ways from the centralized wars of Damascus and part of a very different phenomenon and I'm not certain the Franks of this period possessed the resources to outright conquer Spain from them once the Arabs were established - especially considering Western Christendom was pretty much on the defensive through this era.
Basically the question you're posing can't be easily answered short of writing a whole carefully researched timeline, that explains how Arab raiders conquer Italy and incorporate it into the Ummayad Caliphate in the first place. Then you can start looking at the ramifications.