> What I find even more laughable is that Serbia could have been the majority player in the region if they weren't just doing imperialism

All the Balkan states (Serbia is Balkan as well) are much smaller, and would end up being economically subsumed by larger neighbors like Greece, Romania, Germany, Italy, Türkiye, and Russia. This is already the case across the Balkans today, and nothing would have changed if Serbia were less belligerent.