I haven't seen any military analyst listing that as a serious possibility. 100k Russian troops could conquer that area, but would be insufficient to control it later. That area is heavily populated with a Ukrainian majority. More credible is creating a Russian land bridge to Crimea which would also solve their water issues.
I would split Ukraine. The western part, which was traditionally a part of the Austro-Hungarian Empire and then Poland, should be an independent State, based in L’viv, free to join the EU and NATO, if they wish. I believe even Putin said this would be acceptable.
Then the main part of Ukraine, based in Kiev, should be neutral, with Donbass and the Russian-dominated areas, free to either be absorbed into Russia, or be an independent State aligned with Russia.
Southern Ukraine south of the Dnieper is not heavily populated. That is the land bridge. And Putin will get the water flow to Crimea again. He will hold the land by threatening Kiev with the Belarus troops. And I think he will take and keep Odessa with the troops near Moldova. Odessa will be easy to cut off from the rear. Putin is following WW2 playbook.