As the war is in a new phase, this seems to me to be a reasonably comprehensive and objective of why its focused in Donbas. Yes its from CNN. So sue me.
https://edition.cnn.com/2022/04/15/europe/donbas-region-ukraine-war-russia-explainer-intl/index.html
Donbas, Ukraine's ravaged heartland, has suffered eight years of warfare. Here's why Putin wants it