I was feeling masochistic so I browsed 
for a while, and remembered that all pro-Ukraine folks seem to take this as a fact. I remember trying to look into this and not finding any evidence, and angry libs were not exactly forthcoming with sources themselves.
So besides the initial feint towards Kyiv (which AFAIK is circumstantial at best) was there ever any evidence pointing towards some Russian plan to eventually annex the entirety of Ukraine?
No. And the Kiev operation is not any kind of evidence, not even circumstantial.
However war goals do change over the course of a war and it may well be that this becomes a goal eventually if the West continues to categorically refuse to accept its defeat in Ukraine and tries to keep propping up a Nazi puppet regime in whatever remains of Ukraine when the Russian speaking parts have been liberated. Annexation may become a strategic necessity to ensure Russia’s safety.