I will have to disagree here.
1)If the A-H keep the Turks and Romanians out of our hair and hit the Serbs and force them to divert forces away from the South-Eastern fron then that means we can finally kick their asses.
As you can see here, our army was
almost twice that of Serbia.
And the Greek army was even smaller.
<Insert meme That's the gay flag meme from Family Guy here.>
2) I think this might make the Austro-Hungarians more cautious about Serbian terrorists down the line and weaken and damage the Serbs to a point where their fantasies for a greater Serbia and Yugoslavia will have to be put on the back burner.
3) A stronger Bulgaria with good relationship with A-H will be a nice big dagger pointing at Serbia's abdomen and could serve as a foil to any and all attempts to play dumb games with A-H, since we will be more than happy to go in there and kick their treacherous asses.
We could also aid some of the other anti-Serbian Slavs thet make up the then Serbia as well as the Bosniaks and the Albanians.
4) This will damage the Pan-Slavism thesis in Russia and weaken it's supporters, especially if we are victorious and we start reminding everyone hoe this is the third or forth time those assholes have backstabbed us.
Our relations with Turkey might be better, too.
So at minimum WWI will start later and get triggered by a different event.
Similarly, once the other threats on the board are mirigeted we can kick the shit out of the Greeks and we keep Solon.
The big questions here is how will the Russians react, obviously it will be preferable if they do not simp for Serbia, ana maybe this timeline will finally teach them not to.
I think the best goal for Bulgaria would be a return to San Stefano borders, and maybe ceding some of the south eastern BW1 gains back to Turkey in exchange for improved relations.
My personal preference would be for Bulgaria and the Austro-Hungarians to just partition the place and then we deal with Serbian idiocy the well known Balkan way.