You think it's easier to invade a country in home territory, while they had years to prepare between the previous invasion and now, than it is to defend against some attack that Ukraine might hope to mount against Russia? That sounds... unlikely. It's apparently also what supporters of Russia are saying, so I doubt this is the whole truth, even if there were a grain of truth to an all-out attack being "self defense".
Of the three options of don't invade, invade a little, or invade the whole country, invading a little would allow Ukraine to mount an attack against the breakaway regions and the West was bound to supply advanced weapons to make sure they would be successful at it. So when Putin said they were going to secure the breakaway regions my first thoughts was that there would be no way to do that unless you neutralize the entirety of Ukraine.
