> But if that's just a pretext, what's the actual reason? I haven't seen a good answer yet.
Because in Putin's mind, "Soviet" and "Russian" are synonyms. In other words, the breakup of the Soviet Union in 1991 was to him the breakup of Russia, and as a Russian nationalist he sees that as a great tragedy. He blames it on the Communists for adopting a constitutional structure which contained within it the seeds of separatism; the Communists only ever meant it as lip-service, and as long as they remained in control that was all it was, but as they lost control those seeds began to sprout and 1991 was the result. To him, Lithuanians, Ukrainians, Georgians, Armenians, etc, are not neighbouring foreign nationalities, they are Russians, they are traditional ethnic minorities of the Russian nationality, just as he views Chechens, Tatars, Tuvans, etc, as being.
He wants to "Make Russia Great Again" by reversing the breakup of 1991, or getting as close to that goal as he possibly can. And annexing large parts of Ukraine and installing one or more puppet governments in the rest is, in his mind, an important step towards that ultimate goal.