You don't believe for a second that the war has always been about the Donbass? Well, it is the region that has been at war for 8 years. It is the region that had Russia's primary focus to start the war. It is the region Russia established quick control of with brutal force. It is the region where Putin said his "special military operation" had to happen.
It is also the region that has a high percentage of that wheat we keep hearing feeds the world. And most of Ukraine's gas reserves. And access to the black sea oil reserves. And the ports Putin snatched to ship those resources and for naval purposes. And the all important land bridge to Crimea. Control of this region (and it is expanded a bit from just the Donbas) cripples Ukraine. The winner of the battle for Donbass, the biggest battle in Europe since 1945, wins this war.
Sure Putin wants to destroy the government and reunite Ukraine to Russia. He'd like that for Georgia, Moldova, the baltics, all the old soviet bloc states. And he is especially obsessed with Ukraine for decades. But he takes bites out of these places. He took Abkhazia and S Ossetia from Georgia. He grabbed Chechnya. He took Transnistria from Moldova. Crimea from Ukraine. It is his MO and this war is about the Donbass and that land bridge to Crimea.