I don’t think Ukraine as a ‘buffer’ means much anymore in the age of thermonuclear warfare. But Crimea has somehow sacred sense, sure. A sort of Russian Alamo. Also Turkish slavers’ raids finished after capture of Crimea. Taking it was a sort of cementing the ‘Southern frontier’ meaning Russia only needs to focus on Western flank to feel safe.
“A sort of Russian Alamo.”
A perfect characterization.
Russia would, and has, undertaken existential war to take it and keep it.