Ah yes, the Grozny, "level everything in sight" option. Well, Chechnya has a population of 1.4 million - and about the size of Connecticut or New Jersey - it's also entirely within Russia's own borders, making the logistical re-supply by rail that the Russian army relies upon quite easy.
You can't do the same thing in a country like Ukraine, which 40+ million and the size of Texas, and where Russian rail re-supply isn't an option. Logistics of transporting artillery aside, Russia literally doesn't have that many conventional bombs and missiles. The supplies are running short. Ukraine's also being resupplied by the West in a way Chechnya never was.
The war could not have been won in a day. Obviously, if Putin could have, he would have.
If Putin wants to flatten Ukraine with nukes or chemical/biological WMDs, he can. But that's what it'd take - and the spillover of these WMD toxins into NATO countries would likely be viewed by the latter as a declaration of war.