Why would Putin need to do that? Is everything not going according to Putin's Cunning Plan™? I thought the Russians had enough regular forces to crush Ukraine several times over, and now Putin has to both call up 300,000 reserves, and cajole his White Russian satrap into chipping in 120,000 Belarussians?
So, you recognize, although you will not admit, that Putin miscalculate on a massive scale. Is it safe for you to do that? With the FSB I mean? I don't think Putin is in the mood to tolerate lèse-majesté on any scale right now, and ripped out fingernails don't always grow back you know.
Perhaps you should sprinkle your posts with some suck up phrases.
Putin, father of the country, and of several students in Swiss private schools.
Putin, he hasn't killed nearly as many of us as Stalin.
Can you imagine Boris Yeltsin riding horseback with no shirt?
Cunning plan? The Russians came to us in December to try to prevent this war. All they wanted was a promise Ukraine would remain neutral and the Minsk Accords, which Ukraine signed off and would have protected ethnic Russians in eastern Ukraine, implemented. That you don't know shows how little you know about the cause of the war.
But, to answer your question -- the war on the ground has mostly been fought by militias of LPR and DPR, Chechens and the Wagner group. Russia has only used 10% of its military on the ground.
Now, however, that the annexed areas are Russian territory, the Russian military will defend it as Russian territory. And, that means the full force of the Russian military will involved.
Ukraine has no chance whatsoever. It tried an offensive in Kherson where the Russian military was and was slaughtered. Ukraine's only "successful" offensive have been in areas the Russians pulled out of.