Well, Bulgaria will clash with Greece over the rest of Macedonia. The biggest prize for both countries is the same: Salonica. In any case, just due to the geography and ethnic makeup of Macedonia (and Thrace as a secondary prize) Greece and Bulgaria are bound to clash. The only case where these nation-states won't clash is if you give Bulgaria a substantially different ethnogenesis. However, the POD is too late for such development.Any Greek support for a Serb domination of land that is plainly Bulgarian, like North Macedonia, is going to be make Bulgarians and Greeks enemies over a territorry that is of no interest to the Greek National Cause.
That means that Greeks and Bulgarians will be enemies in any case. So Greece can either seek allies or isolate herself diplomatically.
Other than the aforementioned diplomatic isolation of Greece there is another thing to consider in this case: There is no Austria-Hungary anymore. The main serbian targets northwards is the ottoman Bosnia. The now weak Triple Monarchy of Austria-Czechia-Croatia cannot hope to snatch the whole province. In the coming decades, a lot of the diplomatic capital of the Habsburgs will be spent in Germany and Italy. Moreover, the Croats enjoy quite a bit of autonomy in the new state. In OTL they were very loyal to the Double Monarchy and jumped into the Jugoslavia idea only after said state was collapsing. Moreover, in OTL they were included in the Lands of the Crown of St Stephen with all the drawbacks of magyarization and a much reduced voice in Budapest. Now their role in the multi-national state is much more important and they won't face a germanization.Better to pull Bulgaria away from Russia, and direct Serbia Northwards
As I see it, the Triple Monarchy is not a natural enemy as was the OTL Double Monarchy. Instead it is a potential partner. Both countries view Hungary as a rival: Burgenland and Slovakia will be targets of Austria and Czechia. Not to mention that a number of Hungarians might want to get Croatia back. Serbia wants to reunite with the Serbs in Banat.
What I see as the most plausible scenario, is Serbia and the Triple Monarchy carve up between them Ottoman Bosnia and become partners when it comes to their main regional rival: Hungary. However, if there will be no Bosnia Question, then a slice of Banat will have less value than what lies in the south: Kosovo and the Vardar valley.
To allow Serbia to have N. Macedonia is also to give credence to Pan-Slavism in the Balkans, something which is also against Greece's interests in the long run.
Actually it is the exact opposite: Panslavism is an ideology of a brotherhood and unity of slavic nations. If the two slavic nations of the Balkans are indeed friendly, then TTL's Panslavism will be orders of magnitude stronger than in OTL and a mortal danger for Greece.
In contrast, if the two slavic nations have clashing interests then by definition Panslavism is weakened.
Overall, a friendly relationship between Bulgaria and Serbia is the worst possible outcome for Greece.
A more powerful Greece will also have less of a need for a knit-tight alliance with Serbia like OTL.
A more powerful Greece will have to deal with an Ottoman Empire/Turkey that in any case will be more populous than Greece. At the same time, Greece will have to deal with a strong Bulgaria. The more powerful Greece becomes by encompassing Macedonia south of the Iron Gates and Thrace south of Rhodope, means that they control more and more of bulgarian targets. Thus, the bulgarian resentment grows. Therefore, a more powerful Greece needs Serbia as facing on her own the combination of Ottoman Empire and Bulgaria will be challenging.
Why ?Serbia will probably fall under the Russian Sphere of Influence too in the long run
As I see it there is almost no chance that the Russians will need Serbia. In OTL there was Austria-Hungary as the regional rival. A rival that controlled Galicia with Ukrainians developing a national consciousness and that was a Great Power with clashing sphere of interests. The clashing spheres of influence do not exist in this TTL. Galicia is part of Russia as well.
In contrast, Hungary is diplomatically isolated with almost every neighbor wanting a part of them (Triple Monarchy, Serbia, Romania). Why I say almost? Russia is the one neighbor with no irredentist claims in Hungary. Instead, it is in russian interest for Transylvania to stay hungarian as a romanian Transylvania will fuel romanian nationalism and will make Romania too big a country to be a good, well-behaving satelite. Romanians might even get the notion of being a trully indepedent country. Russia would shudder at that.
So, Hungary - a potential serbian rival, not only lacks the means to pursuit a sphere of influence in the Balkans, but also is in need of russia help to keep their current territory intact.
Overall, there are precious few reasons for Serbia falling under the russian sphere of influence.