The AFU collapsing seems like the more probable scenario because the extreme right wing is the main political force in Ukraine right now. So, even if Zelensky were removed, it's almost certain he'd be replaced by somebody who'd continue the war. The nationalist battalions being broken will end the war, however.
The most likely scenario, in my opinion, is that Russia will run referendums in all the oblasts that are pro-Russian or neutral and absorb them. Then, they're going to leave a rump western Ukraine that's not going to be viable as a problem for the west to deal with. It's going to be a huge economic burden on Europe because if it's allowed to fail, that will result in a massive refugee crisis at a time when the European economy is hanging by a thread. And the alternative is to keep pouring money in to prop it up, making it an economic black hole. It's also possible that Poland, Hungary, and Romania decide to carve up what's left of Ukraine.