If you want to know his mind, you just have to listen to/read what he's been saying for basically two decades about Ukraine, Russia, and the illegitimacy of the dissolution of the Soviet Union more generally.
Just last summer, Putin wrote an essay regarding Ukraine, basically mocking the entire idea of Ukraine existing as a nation at all. One of the things he said was:
"true sovereignty of Ukraine is possible only in partnership with Russia.”
The idea that Putin's interest in Ukraine is limited to protecting a few border areas and security is 't based in reality. He either wants all of Ukraine as part of Russia, or a Ukrainian puppet state controlled by Moscow.
I agree he probably wants a puppet state or a very friendly and cooperative government there. I do get why he doesn’t want a west aligned country there. The corruption in Ukraine is off the charts, I wonder if that’s why the left is so worried about their dealings there that they’re willing to risk nuclear war.