The problem is that OTL Ottoman capitulations first were signed by Suleiman the Magnificent from the position of power. You only need an overconfident Emperor to have the same thing occur.
Greeks/Turkish/Armenians/Bulgarians/Kurdish/Serbians/Slavs/Arabs are distinct people with distinct religions/cultures/languages and with sufficiently significant population... so logically Roman Empire should become a hotbed of early nationalist/separatist movements. In addition, Rhome has lots of hostile neighbours and enemies due to its strategical geographical position.
But, I will wait and see and watch how things will play out for the Romans ITTL. Most likely it will end well for them in this specific TL.
I presume that you're talking about Suleiman's failed siege of Vienna. The reason he agreed to peace was that he had seen that he was unable to take Vienna with the resources that he had. So he retreated, to develop more resources for a future strike at Austria. Overextension was a thing. The problem was that Suleiman's successors were not as good as he was, so they were not able to see that a plunge into Austria would overextend the empire too far.
ITTL, that Suleiman era was Andreas the Victor. His successors (especially Alexeia and that sonofab----h advisor Fyodor) were unable to hold those conquests and had to fall back. Except the Sideroi are trying to fix it. The German war is just a big raid into Bavaria for revenge. And don't dare tell me that a Roman Empire without even Serbia is overextended. Furthermore, the Sideroi have created a safety net of allies to back Romania up, whereas OTL Ottomans were alone.
Now firstly, the Empire has very few Serbs and non-Bulgarian Slavs. Secondly, all of these people except the borderland Armenians, Kurds and Muslim Arabs are thoroughly integral parts of Roman society. This is not the OTL Ottoman Empire, where Turks ruled everything, a Muslim group was lording it over Christian groups and non-Turkish culture was not encouraged. Roman society is a curious mishmash of all of them. It is inclusive, not exclusive. (Unless you happen to be a Muslim Arab...)