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.
Your post makes two assumptions. The first is Putin wants to end the war in Ukraine. The second is the only war Russia is fighting is in Ukraine.
Russia is waging a much larger economic and psychological war against the EU, the U.S., and NATO. The kinetic, conventional war in Ukraine is the catalyst for the larger war. If the war in Ukraine ends, Putin loses the lever he is using against the NWO. Add into that, he still wants to capture Ukraine in as good as shape as he possibly can.