Putin has no reason to roll into Ukraine proper. The purpose of instigating a sessession in Donbass was to distract from and seal the Crimea takeover, while eliminating any prospect of potential NATO membership by creating a territorial conflict. His greatest opportunity to take the rest of the country was right there, in 2014 when it was drown in chaos and there was little to no physical threat in the way and also nearby. The UA may still not be any major threat to the Russian military and I doubt NATO would intervene directly and waste it's NRF and whatever air assetts it can scramble in Europe, but it's barely the same situation as back then when nobody expected it and wasn't ready. He is also risking to bleed out his military in a large scale conventional war and subsequent resistance if the Ukrainians refuse to give up their country. It may not be the same as fighting an insurgency in the North Caucasus as people there can be extremely vicious but if they were forced to, well Ukraine is a much bigger fish to deal with.