I also don’t think they will annex entire Ukraine. There’s the issue of Ukraine not being unified country. I mostly wonder if Russia will try for Odessa and for the full Black Sea run. It’s probable because that part of russian interest didn’t changed since at least Peter I. That would also give them land border with Transnistria. And would leave Ukraine as only Galicia rump state, probably unstable and full of terrorists. Thing is, this instability and terrorism would in very large part be aimed at Europe and be buffer state - but on the other hand, Russia might be up for the challenge anyway since no one would want such buffer state at the borders, and Ukraine sowing terror across Europe is in the USA interest as well.

Also it all depends on how the war would end, since wars have nasty habits of not going according to plans.