I would also like to add something: the Bulgars historically were close enough to Constantinople and under the constant watch of the emperors, yet it was only during the IX century that they converted to Christianity and only during the reign of the latter Macedonians, that the fight turned in favour of the empire. All of this despite the disparity in resources between the two powers. Here Thrace would be a peripheral borderland of an already overstretched Caliphate. Plus, the Arabs would at least for a while be busy subduing Anatolia, which was the real core of the empire, its armies being the ones that made and unmade several emperors. Constantinople here would be what Narbona was for the Franks, A Muslim enclave in an otherwise entirely Christian/Pagan theatre. And while this would no doubt be a terrible blow for the empire, it would not necessarily mean its end: first you have the Anatolian Strategoi who would most likely appoint one of their own as emperor in the East. In the West you would also see a breakaway roman state based in Syracuse or Ravenna (which almost happened in OTL). Such empire would be in control of the Western Mediterranean islands (Sicily would likely be its new core), Rome, Ravenna, bits of Southern Italy and Dalmatia and with some luck some coastal fortress in the Balkans (pushing roman luck a bit we could also include maybe Crete and Thessalonika as an enclave). Of course, this empire would have way less resources than its OTL counterpart, but its not like Constantinople spent a good deal of them protecting its western holdings in OTL. And yet it took more than a century for the Muslims to take Sicily. Benefitting from renewed imperial focus and commitment in the region, Italy would be a tougher nut to crack for both the Lombards and the Arabs. But history could really go either way, from the Romans being entirely wiped out before the end of the century, to both empires making progress in restoring imperial authority in Italy and the Balkans.