Putin should threaten to drop a tactical nuke on Kyiv in 7 days if Ukraine doesn't surrender. If they still refuse to negotiate after Kyiv is nuked then he can threaten to nuke Lviv next. This would give civilians enough time to leave the city.
The US nuked Japan twice to end a war, and that was without any warning. If Putin gave a week's notice that would be enough time for everyone to leave the city to minimize civilian deaths.
IMO, from Putin POV, this war is already over.
Russia already captured the desired territories, completed the referendum (probably fake, but doesn’t matter for optics), declared the regions as part of Russia and pulled back their forces into defensive positions.
Ukraine now has to undo the above actions, including the referendum to match the optics.
Ukraine’s is now bombing their own/prior territory and is calling that as winning.
NATO was so focused on helping Ukraine, it is now in a pickle – it is helping Ukraine to bomb it’s own/prior territory.
Russia will keep their defensive stance until NATO gets tired of it’s support to Ukraine in their own economic interests due to recession.