It was instrumental for every reason I just listed

Their leadership, when their lines were not overstretched and resources split, could perform well enough. Certainly well enough to fight the Russians who were the worst combatant in a scenario where the Austrians have their full army.

Except no. Italy joining the war significantly altered the war. Austria was now doomed to gimp forever. The Austrian army, once it sorted it's kinks out, was performing as well as the other armies, however it had a severe lack of supplies and didn't cannibalize it's economy as much as Germany to make up for it. They were also overstretched on every single front due to the Italian entrance. Italys entrance meant 2 million Austrians be away from the eastern front. If Italy doesn't join Austria can actually perform well, throwing it's full weight against Russia. They'd be able to perform well. All their resources and manpower on one front. No longer overstretched, actually having the manpower to mount offensives alone. Something as devastating as the Brusilov offensive becomes impossible for the Russians in this TL, the eastern front is held by mostly Austria with minor German support. Germany is able to throw more divisions towards the western front, they win at Verdun, they push back the British at the Somme.

Summer of 1916 was a horrible time for the CP, they were this close to winning at Verdun, but the Brusilov offensive devastated the overstretched Austrians, to the point of collapse, forcing Germany to halt their near victory at Verdun and patch up the eastern front. Then the Somme happened. By January 1917 the CPs manpower situation was dire, all because every single attack they had made in 1916 had failed. And those failures can be linked to the collapsing eastern front.

If Italy stays neutral, then Austria with minor German support is able to take on the Russians on their own, who are incapable of mounting any meaningful offensives against the Austrians. In 1915 the CP pushed up to Minsk, the Austrians and eastern German leaders wanted to go further, but Falkenhayns plans for Verdun didn't allow that. Now they can push further, Petrograd likely falls by 1916, Russia pulls out of the war by late 1916 or early 1917. Germany this entire time has had a strengthed western front, as I said they win at Verdun and push the British back at the Somme. In OTL they had 2 million men in the eastern theater, they pulled 900,000 out for the western front offensives in 1918. In this TL they don't need 2 million men in the eastern front, they likely have the full 190-200 divisions on the western front, which is what they attacked with in spring 1918. Germany can now launch the massive offensives it launched in 1918 but significantly earlier, likely spring 1917. But this time their logistics are significantly better because German supplies haven't been wrecked by another year of war. Oh and Italy staying neutral means Austrian ports are open, making the British blockade of Germany worthless. That means no unrestricted warfare that brought America into the war. Anyway, Germany launches their offensive but with better logistics, likely breaks through the British lines as they did in 1918, but this time they have the logistics to take Amiens. Splitting the British and french armies, at which point the Entente sues for peace. A peace favorable to the Central Powers.

You don't just discredit the contribution of millions of men.