I don't think it's possible unless some things beyond the Empire's control were to happen. Maybe the Magyars are more stronk in this timeline And absolutely devastate the Franks and Lombards while somehow not going after anything Byzantine for some reason, maybe Persia is irrevocably weakened and turned into a Roman client state through some combination of unlikely but not impossible circumstances. Maybe the Alchemists who made Greek fire start to diddle around with proto-gunpowder. Maybe the Franks stay Arian? You probably know better than me with all the timelines you did.
Yeah but a supply line that Runs along the border doesn't sound the most stable to me. A canal was built in Charlemagne's time so it's fairly possible.
My main point was that we’ve got a millennium and a half to play with - look what an offshoot of an offshoot of an offshoot of a tribal group that had barely formed at the time (the Ottomans) did with the resources of the Byzantine east, after having rebuilt their borders basically from scratch, in just a few hundred years.
Here, Byzantium has a better starting point, a long standing legacy in the bulk of the territory in question, and vastly more time. They can wear down the Lombards, then wait for the Franks to crumble, even if it takes waiting until the Norse invade - Byzantium is wonderfully positioned to pick up the pieces there. Especially if they’re doing well, it will be taken for granted that of course they’re going to restore the old borders, even if takes them a few centuries.
Meanwhile, I wasn’t thinking of the Danube as a supply line so much as a line of communication and troop movement - an Empire can still source its supplies from local depots. Beyond that, I’m arguing that, with the time available to them and the increase in development of Europe beyond the Rhine-Danube frontier, Byzantium would have every reason to push further outward, so the Rhine-Danube wouldn’t be a frontier anymore - it would be a key spine for the military and economy of the empire.