[Is the US going to be able to deter and contain Russia and China? Ominously, both have embraced revanchist thinking as a basis for otherwise threadbare regime legitimacy. This has stoked popular sentiment favoring military adventures. Both countries are now bullying their neighbors, acting on dodgy territorial claims, and beginning to project military power far from their borders.]
Alexander was the king of a united Greece. Did he need to conquer Persia? Only if he wanted children and cities (Kandahar in Afghanistan, Iskandaria in Egypt, Iskandariyah in Iraq) to be named after him thousands of years later.
Rulers of great nations are already prominent men. They want first place - not just on the list of their nations’ rulers, but on the list of great (i.e. powerful) rulers spanning all of history. To do that, they need to expand the territory handed to them at the time of their ascension to power. Nobody remembers the rulers who made their citizens rich. Everyone remembers the ones who greatly expanded the borders of their nations. These people are not interested in defense - they’re concerned with cementing reputations that will match Alexander’s or Julius Caesar’s.
In Thucydides’ rationale, the terms glory and self-interest encompass the motives you prefer to rely on for explanation. As for the specific case of territorial expansion, Chairman Xi and his closest allies do not try to justify territorial expansion as a personal vanity project. Instead, they promote it to the regime and the public as part of reclaiming China’s rightful historical greatness. That in turn excites China’s public to rally around the regime in support of military expansion and possible adventurism.