First of all, why in the world would Britain make such a colossal turn of policy towards the Eastern Question? Britain had vested interests in maintaining an Ottoman Empire under its influence to its economic benefit and to prevent Russia from gaining access to the Mediterranean.
Second, how could one reconciliate the ambitions of Russia, France, Britain and Austria towards the Balkans? That's just not doable - politicians wouldn't be able to just draw lines on a map and every Great Power would end up satisfied. Russia will be sure to want effective control of a much larger Bulgaria than the one in your map (see OTL Treaty of San Stefano), a special role in Constantinople if not outright annexation, access to the Mediterranean Sea, Serbia brought under its sphere of influence and increased influence in Greece. Britain would never accept even a quarter of these demands, and it will also want Cyprus as a naval base (OTL was no accident nor did the British just pick a neat looking island in 1878), but it certainly wouldn't accept the loss of influence in the Balkans and Anatolia, which it had before thanks to Greece and the Ottoman Empire. Greece will no longer feel the necessity of English protection and is very likely if not sure to come much closer to the Russians, who in turn will find themselves in a much bigger, earlier and high-stakes Great Game against the British. And that's not even considering the certain rifts to come between Austria and Russia over Serbia, Romania and Bulgaria as well between France and Britain over the Lebanon, Cyprus, Egypt and Cyprus should France lay claim on it. It's a mega-Crimean War waiting to happen, this time involving every OTL nation as well as all countries between the Danube and Mesopotamia. There's a reason no Great Power really wanted to dismantle the Ottoman Empire and held it together for as long as they could.
And how can you get the Bulgarians, Romanians, Serbians, Arabs and, most of all, Turks (or, more accurately for a pre-Turkish nationalism context, Muslims) to play along? First of all, Bulgaria couldn't be made a nation-state just like that, as this was before the 1870s, the establishment of their special millyet in 1860 and the 1841 revolt which pivoted Bulgarian nationalism. You also now have important Serb populations in Greece and that wouldn't be good, not to mention Serbians didn't just fight two wars to become puppets of the Habsburg monarchy. As for Arabs, that has been covered already. Romanians won't pose difficult problems but long-term, they will resent Russian control. Last but not least, you now have huge Muslim minorities across the Balkans long before resettlements were anywhere close to being considered a solution to demographic homogenisation of countries. And it would be interesting to see what happens to all those factions fighting a civil war for the throne of the Sultan once they are faced with an ultimatum, if not fait accomplit, to abandon Istanbul and 2/3rds of the Empire, to resettle where exactly? There can be no Ottoman state without Constantinople and there can be no Turkish state without Turkish nationalism and some kind of awakening. So what exactly is the country that exists in Anatolia, apart from majority Muslim and comprising of a myriad of ethnic groups with no cohesive national narrative and a very large Christian minority?
Lastly, how can one achieve a non-disastrous scenario for Greece as you're making it like 3 to 4 times as large as it was before (assuming a POD before 1912) with large to huge minorities north of Thessaly and Anatolia if it's given the coast or more, whilst keeping hundred of thousands of Greeks outside Greece? The country had just come out from a devastating War of Independence, three civil wars, political strife, with a bankrupt economy, a population in dire condition, a foreign incompetent King and the Megali Idea was not even existent back then. If you just give it so much land the entire thing will collapse on its own spectacularly.
