Many feel that Russia no longer have the ability to take major cities. I disagree. If you look at the speed in which cities are taken. From Mariupol onward, it has been speeding up. Lyman only took a couple of weeks, Severodonetsk only took a few days(if you don't count the counter-attack which is militarily suicidal). Some think that Russia suffered up to 30k losses. I disagree. The most recent fighting has been mainly Russia killing Ukrainians with cannons, then fighting in the city, in which the Ukrainians seems to collapse pretty quickly. There was no storming of buildings. The Russians mainly send in troops as scout, calling out firing positions for their aircrafts and cannons. My guess is 4000 dead and 3x that wounded. Maybe 16K out of action altogether.
After Donbass, Russia will have all of its military at their disposal to take cities. They can take the cities one or two at a time. With their major arm forces gone from Donbass, Ukrainian defense for a given city will be significantly weaker compared to their Russian counterparts. Maybe 3 to 1 advantage. This is something that did not exist in the siege of Mariupol or other cities. Since the Russians are not storming the cities with bodies, but mainly to have people scout out firing positions for their cannons, they don't really need the 3 to 1 advantage. There will be no stopping them if they want to take any city. For this reason I voted for 6. I did not vote for 7 because the cost of holding Western Ukraine is not worth it for the Russians, plus they want to create a place for all those who are anti-Russian to go. It make sense to keep a small region for these people to go to. In the longer run, a landlocked rump of Ukraine cannot support a large population since it lack any industry, so most will chose to migrate to the West. Even if the NATO wanted to organize an army, the five people that are left in that region would pose no danger to the Russian part of Ukraine.