The problem is if only half of Ukraine is taken, the other half will be turned into a terrorist kamikaze state by NATO with sole reason of existence is to hurt anything Russian. Yes, even worse than now, it will be literally like ISIS. I don’t know about opening other fronts but Russian forces are severely outnumbered because of special operation format but since for Russia winning is a matter of existence, if thing will go really, really wrong, Putin will announce mobilization. So far Russia is only fighting on territory very close to border where support of locals is high.