As others have noted, the Umayyads were damn lucky to have managed as much IOTL, especially when one considers that the rapidity of the Muslim conquests left large areas (Sistan, Tabaristan, Armenia) nominally subjected but in reality away from Muslim control until well into the Abbasid period. With hindsight, the Umayyads could have done a few things differently. First is not try to expand in all directions at once, but prioritize. This would definitely have made all the difference under Hisham, when the Syrian army, the pillar of Umayyad rule, found itself parcelled out to support literally all the buckling frontiers. However, as jihad was built-in in the system, and as the Umayyad system required the booty from expansion to keep the Arab soldiery quiescent, it is difficult to see what could have been changed, without the hindsight we have. Second, and possibly the most important, is not to try and create an Arab apartheid, but actually accept the mawali (especially the Berbers) as equal partners in the "Caliphate" enterprise. This would have increased the regime's stability, and provided more manpower. It is arguable though that, as others have pointed out, for logistical reasons in many areas the Umayyads had reached the maximum limit. Beyond lay lands that could be conquered, but were unprofitable to do so-like the Romans and Germania in the 1st century. This was the case in the Caucasus, Central Asia, and probably Iberia-Gaul as well. Byzantium could have been destroyed with a more focused approach to wear down its defences in Anatolia, but the terrain there favoured the defender. A more naval approach IMO holds more promise, focusing on Sicily and Crete, and the soft underbelly of Europe, but this requires a major shift in the strategic outlook of the Umayyads.